This PDF file contains the front matter associated with SPIE Proceedings Volume 7739, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.
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Large aspheric primary mirrors are proposed that use hundreds segments that all must be aligned and phased to
approximate the desired continuous mirror. We present a method of measuring these concave segments with a Fizeau
interferometer where a spherical convex reference surface is held a few millimeters from the aspheric segment. The
aspheric shape is accommodated by a small computer generated hologram (CGH). Different segments are measured by
replacing the CGH. As a Fizeau test, nearly all of the optical elements and air spaces are common to both the
measurement and reference wavefront, so the sensitivities are not tight. Also, since the reference surface of the test plate
is common to all tests, this system achieves excellent control for the radius of curvature variation from one part to
another. This paper describes the test system design and analysis for such a test, and presents data from a similar 1.4-m
test performed at the University of Arizona.
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Optics for telescopes - on ground and in space - is getting more and more into complex geometries. Weight reduction
and new materials together with aspherical shape and off-axis set-ups increase the need for deterministic processes. With
the advent of free-form surfaces having no symmetry at all, a new chapter for fabrication issues is opened.
This paper describes our current achievements to combine different fabrication and measurement technologies to cope
with the increasing demand in precision and complexity. We will explain our fabrication approach covering the full
range from the raw material to the coated and measured component. Several examples of current and recent projects are
shown. The variety of materials used ranges from Zerodur® to SiC.
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An ultra precision large optics grinding machine, BoX®, was developed and produced at Cranfield University. BoX®
offers a rapid and economic solution for grinding large off-axis aspherical and free-form optical components. Grinding
high accuracy surfaces with low subsurface damage reduces subsequent polishing time. This efficient grinding process
provides the capacity to grind 1.5 m parts. This paper presents an analysis of Astrositall® optical ground parts: a
hexagonal 84 m radius of curvature mirror of 1 m across corners and an off-axis 350 mm diameter mirror. The 1 m
hexagonal part is representative of segments under study for making extremely large telescope (ELT) segmented mirrors.
The second part was machined off-axis to demonstrate free-form fabrication capability. These operations demonstrate
the scalability of the rapid grinding process developed for large free-form optics. The use of an error compensation
procedure improved an initial ground form accuracy to +/- 1 μm p-v over 1 metre surface. The results highlighted the
effect of grinding parameters and machine dynamics on form accuracy and fabrication time.
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JSC LZOS under the contract with firm AMOS is carrying out the manufacturing works of Primary and Secondary
Mirrors of Devasthal Optical Telescope (DOT) for Aryabhatta Research Institute of Observational Sciences (ARIES).
Primary mirror specifications: diameter is 3700 mm, vertex radius is 14639 mm (F/1.96), conical constant is -1.03296,
asphericity is 111 microns. Secondary mirror specifications: diameter is 980 mm, vertex radius is 4675 mm (F/1.78),
conical constant is -2.79561, asphericity is 47 microns. The current progress status under this project is presented here in
the manuscript.
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Aspherical surfaces for imaging or spectroscopy are a centerpiece of high-performance optics. Due to the high
alignment sensitivity of aspheric surfaces, reference elements and interfaces with a tight geometrical relation to
the mirror are as important as the high quality of the optical surface itself.
The developed manufacturing method, which accounts for the shape and also for the position of the mirror
surfaces, allows controlling and precisely correcting not only the form, but also the alignment of reference marks,
interfaces or even other mirrors in the sub-assembly using diamond turning. For Korsch or TMA telescopes it is
also possible to diamond turn whole sub-assemblies containing two or more mirrors with a relative position error
as low as the machine precision. Reference elements allow the correction of the shape and position of mirrors as
well as the position of interfaces for system integration. The presented method opens up a novel manufacturing
strategy to enhance the relative positioning accuracy of optic assemblies by one order of magnitude.
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The primary mirror of the Giant Magellan Telescope consists of seven 8.4 m segments which are borosilicate
honeycomb sandwich mirrors. Fabrication and testing of the off-axis segments is challenging and has led to a number of
innovations in manufacturing technology. The polishing system includes an actively stressed lap that follows the shape
of the aspheric surface, used for large-scale figuring and smoothing, and a passive "rigid conformal lap" for small-scale
figuring and smoothing. Four independent measurement systems support all stages of fabrication and provide redundant
measurements of all critical parameters including mirror figure, radius of curvature, off-axis distance and clocking. The
first measurement uses a laser tracker to scan the surface, with external references to compensate for rigid body
displacements and refractive index variations. The main optical test is a full-aperture interferometric measurement, but it
requires an asymmetric null corrector with three elements, including a 3.75 m mirror and a computer-generated
hologram, to compensate for the surface's 14 mm departure from the best-fit sphere. Two additional optical tests
measure large-scale and small-scale structure, with some overlap. Together these measurements provide high confidence
that the segments meet all requirements.
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Advanced shapes can now be produced for the corrective optics placed near a reimaged pupil, or even a deformable
mirror surface. These surfaces can be improved and even apodization added to improve contrast. In this paper, we
describe a special form of Narrow Ion Beam Figuring (NIBF) developed at L-3 Tinsley. In contrast to existing Ion Beam
Figuring (IBF) machining schemes, the FWHM beam width is controlled in a much narrower band while still providing
high beam currents.
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Pending critical spaceborne requirements, including coronagraphic detection of exoplanets, require exceptionally
smooth mirror surfaces, aggressive lightweighting, and low-risk cost-effective optical manufacturing methods.
Simultaneous development at Schott for production of aggressively lightweighted (>90%) Zerodur® mirror blanks,
and at L-3 Brashear for producing ultra-smooth surfaces on Zerodur®, will be described. New L-3 techniques for
large-mirror optical fabrication include Computer Controlled Optical Surfacing (CCOS) pioneered at L-3 Tinsley,
and the world's largest MRF machine in place at L-3 Brashear. We propose that exceptional mirrors for the most
critical spaceborne applications can now be produced with the technologies described.
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Nanjing Institute of Astronomical Optics & Technology (NIAOT) is investigating two types of sub-aperture polish
technique for manufacturing aspheric components in large astronomical telescopes. One technique is computer
controlled optical surfacing (CCOS). It removes material by a small polish tool through traditional mechanical and
chemical process. The other is ion beam figuring (IBF) technique. It employs a neutralized ion beam to physically
sputter material form optical surface. Although the basic mechanism of the two techniques is different, they true share
the same mathematical model and fabrication diagram which will be put forward firstly in this paper. Then tool design
and material removal function in CCOS will be studied following by a fabrication instance using CCOS. After that some
recent progresses achieved in IBF is presented. The last part will focus on the complementary relationship of CCOS and
IBF. Using them alternatively optimal combination of surface precision, efficiency and edge control could be obtained.
Simulation is provided to support this view and experiment will be done in near future.
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The stress polishing method is well suited for the superpolishing of aspheric components for astronomy. The main
advantage of this technique is the very high optical quality obtained either on form or on high spatial frequency
errors. Furthermore, the roughness can be decreased down to a few angstrom, thanks the classical polishing with
a large pitch tool. We describe here the results obtained on the three toric mirrors for the ESO/VLT-SPHERE
instrument, dedicated to exoplanet direct imaging. On going work on variable off axis aspherics is presented in
the frame of the ESA/ASPIICS-Proba3 mission for solar coronography and applications for Exoplanet detection
and solar observations are discussed.
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ITT has patented and continues to develop processes to fabricate low-cost borosilicate mirrors that can be used for both
ground and space-based optical telescopes. Borosilicate glass is a commodity and is the material of choice for today's
flat-panel televisions and monitors. Supply and demand has kept its cost low compared to mirror substrate materials
typically found in telescopes. The current technology development is on the path to having the ability to deliver imaging
quality optics of up to 1m (scalable to 2m) in diameter in three weeks. For those applications that can accommodate the
material properties of borosilicate glasses, this technology has the potential to revolutionize ground and space-based
astronomy. ITT Corporation has demonstrated finishing a planar, 0.6m borosilicate, optic to <100 nm-rms. This paper
will provide an historical overview of the development in this area with an emphasis on recent technology developments
to fabricate a 0.6m parabolic mirror under NASA Earth Science Technology Office (ESTO) grant #NNX09AD61G.
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In the last decade a new window for ground-based high energy astrophysics has been opened. It explores the energy band
from about 100 GeV to 10 TeV making use of Imaging Atmospheric Cherenkov Telescopes (IACTs). Research in Very
High Energy (VHE) gamma-ray astronomy is improving rapidly and thanks to the newest facilities as MAGIC, HESS
and VERITAS astronomers and particle physicists are obtaining surprising implications in the theoretical models.
New projects have been started as the European Cherenkov Telescope Array (CTA) and the U.S. Advanced Gamma-ray
Imaging System (AGIS). The aim is to enhance both the sensitivity and the energy band coverage to perform imaging,
photometry and spectroscopy of sources. In this framework, tens of thousands of optical mirror panels have to be
manufactured, tested and mounted into the telescopes. Because of this high number of mirrors it is mandatory to develop
a technique easily transferable to industrial mass production, but keeping the technical and cost-effectiveness
requirements of the next generation of TeV telescopes.
In this context the Astronomical Observatory of Brera (INAF-OAB) is investigating a technique for the manufacturing of
stiff and lightweight glass mirror panels with modest angular resolution. These panels have a composite sandwich-like
structure with two thin glass skins on both sides of a core material; the reflecting skin is optically shaped using an ad-hoc
slumping procedure. The technology here presented is particularly attractive for the mass production of cost-effective
mirror segments with long radius of curvature like those required in the primary mirrors of the next generation of
Cherenkov telescopes. In this paper we present and discuss some relevant results we have obtained from the latest panels
realized.
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Modeling of the thermal expansion behavior of ZERODUR® for the site conditions of the upcoming Extremely Large
Telescope's (ELT's) allows an optimized material selection to yield the best performing ZERODUR® for the mirror
substrates.
The thermal expansion of glass ceramics is a function of temperature and a function of time, due to the structural
relaxation behavior of the materials. The application temperature range of the upcoming ELT projects varies depending
on the possible construction site between -13°C and +27°C. Typical temperature change rates during the night are in the
range between 0.1°C/h and 0.3°C/h. Such temperature change rates are much smaller than the typical economic
laboratory measurement rate, therefore the material behavior under these conditions can not be measured directly.
SCHOTT developed a model approach to describe the structural relaxation behavior of ZERODUR®. With this model it
is possible to precisely predict the thermal expansion behavior of the individual ZERODUR® material batches at any
application temperature profile T(t). This paper presents results of the modeling and shows ZERODUR® material
behavior at typical temperature profiles of different applications.
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Today's space applications increasingly utilize lightweighted construction concepts, motivated by the demands of
manufacturing and functionality, and by economics. Particularly for space optics, mirror stability and stiffness need to
be maximized, while mass needs to be minimized.
Therefore, mirror materials must possess, besides high material strength and manufacturing versatility, high thermal
conductivity combined with low heat capacity and long-term stability against varying thermal loads. Additionally,
optical surfaces need to be compatible with reflective coating materials.
In order to achieve these requirements, the interplay between material properties and mirror design on one hand, and
budgetary constraints on the other must be considered.
In this paper, we address these issues by presenting an FEM design study of open and closed-back mirror structures with
extremely thin reinforcing ribs, with the goal of obtaining optimal physical and optical characteristics. Furthermore, we
show that ECM's carbon-fiber reinforced SiC composite, Cesic®, and its newly developed, HB-Cesic® , with their low
CTE, low density, and high stiffness, are not only excellent mirror materials, but allow the rapid manufacturing of
complex monolithic optical structures at reasonable cost.
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There is growing interest within the Astronomical community in the development and use of very large aperture
telescopes which will incorporate the latest advancements in optical materials. Two of the most notable of these large
size telescope projects, the Thirty Meter Telescope (TMT) and European Extremely Large Telescope (E-ELT), will use
mirror segments in the actively controlled primary mirrors. In this paper we present the results of a material
characteristics study on Ohara CLEARCERAM®-Z HS large diameter blanks which includes data on the CTE, CTE
uniformity, residual stress and internal quality targeting potential use in the TMT Primary Mirror Segment Banks.
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The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective,
three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly,
integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic
vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature
Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA
contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration
capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA
distance. This paper will introduce the systems level architecture and optical layout of the CoCOA and its main
subsystems.
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The 1.5 m primary ZERODUR® mirror of the solar telescope GREGOR incorporates 420 pockets at the backside for
active cooling to avoid the thermal load impact of the sun deteriorating the observation. This design is also under
consideration for the 2 m Indian Solar Telescope and for the 4.2 m European Solar Telescope (EST). The tip and tilt M5
mirror of the European Extremely Large Telescope (E-ELT) requires an even more demanding approach in light
weighting. The approximately 3 m × 2.4 m elliptical flat mirror is specified to a weight of less than 500 kg. During the
successful manufacturing of the GREGOR light weighted mirror, SCHOTT developed a systematic approach for
processing such complex and long lead items which are capable for being up-scaled to a dimension of 4 m. In parallel
SCHOTT has tested the machining of challenging aspect ratios of rib thickness and pocket height to prove the
machinability of the E-ELT M5 design suggestions. The improved data on the bending strengths of ZERODUR® enable
aggressive designs for light weighted 4 m class mirrors.
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The Giant Magellan Telescope has a 25 meter f/0.7 near-parabolic primary mirror constructed from seven 8.4 meter
diameter segments. Several aspects of the interferometric optical test used to guide polishing of the six off-axis
segments go beyond the demonstrated state of the art in optical testing. The null corrector is created from two obliquelyilluminated
spherical mirrors combined with a computer-generated hologram (the measurement hologram). The larger
mirror is 3.75 m in diameter and is supported at the top of a test tower, 23.5 m above the GMT segment. Its size rules out
a direct validation of the wavefront produced by the null corrector. We can, however, use a reference hologram placed at
an intermediate focus between the two spherical mirrors to measure the wavefront produced by the measurement
hologram and the first mirror. This reference hologram is aligned to match the wavefront and thereby becomes the
alignment reference for the rest of the system. The position and orientation of the reference hologram, the 3.75 m mirror
and the GMT segment are measured with a dedicated laser tracker, leading to an alignment accuracy of about 100
microns over the 24 m dimensions of the test. In addition to the interferometer that measures the GMT segment, a
separate interferometer at the center of curvature of the 3.75 m sphere monitors its figure simultaneously with the GMT
measurement, allowing active correction and compensation for residual errors. We describe the details of the design,
alignment, and use of this unique off-axis optical test.
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The most challenging of the metrology needs of multi-objects instruments is the registration of the pupil on the
deformable mirror which corrects the wavefront errors. Pick-off mirrors in multi-objects instruments and specially
spectrographs (MOS) require accurate positioning and simultaneous viewing of the pupil on the deformable mirror
(DM) and the focal plane image on the image slicer at the sub-micron level. A laboratory test prototype simulating the
telescope (E-ELT), the beam steering mirror (BSM) and the pupil imaging mirror (PIM), is presented to confirm the
correct positioning of the pupil on the DM and to provide the movements of the moveable optical elements to achieve it.
The opto-mechanical design and testing of this prototype is shown. The BSM stages (Goniometric cradle, Rotation, &
Linear) provide the key mechanical system elements, with precision alignment, resolution, and repeatability .
The design and behaviour of the control system is discussed; the ultimate aim of which is to adjust the BSM and PIM to
correct for any slight mis-positioning of the pick-off mirror and any temporal drift of all the components to achieve the
required alignment. The control system can also cope with flexure effects when required.
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Since last years and at present days LZOS, JSC has been producing a range of primary mirrors of astronomical
telescopes with diameter more than 1m under contracts with foreign companies. Simultaneous testing of an aspherical
surface figure by means of a lens corrector and CGH (computer generated hologram) corrector, testing of the corrector
using the CGH allow challenging the task of definite testing of the mirrors surfaces figure. The results of successful
figuring of the mirrors with diameter up to 4m like VISTA Project (Southern European Observatory), TNT (Thai
National telescope, Australia - Thailand), LCO telescopes (Las Cumbres Observatory, USA) and Russian national
projects and meeting these mirrors specifications' requirements are all considered as the sufficient evidence.
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The construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005. At the
beginning of 2006, it was realized that the telescope's image quality suffered from optical aberrations, chiefly a focus
gradient across the focal plane, but also accompanied by astigmatism and higher order aberrations. In the previous
conference in this series, a paper was presented describing the optical system engineering investigation which had been
conducted to diagnose the problem. This investigation exonerated the primary mirror as the cause, as well as the science
instruments, and was isolated to the interface between the telescope and a major optical sub-system, the spherical
aberration corrector (SAC). This is a complex sub-system of four aspheric mirrors which corrects the spherical
aberration of the 11-m primary mirror. In the last two years, a solution to this problem was developed which involved
removing the SAC from the telescope, installing a modification of the SAC/telescope interface, re-aligning and testing
the four SAC mirrors and re-installation on the telescope. This paper describes the plan, discusses the details and shows
progress to date and the current status.
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This paper will focus on the metrology of multiple complex surfaces that are to be integrated into the KBand Multi-
Object Spectrograph (KMOS). KMOS is a multi-field astronomical spectrograph designed for integration with the 8.2m
diameter European Southern Observatory Very Large Telescope (VLT). There are 1080 separate optical surfaces in the
design, many of them complex freeform surfaces. Optical surfaces were manufactured in aluminium by precision
freeform diamond machining. This flexible technique allows the fabrication of extremely complex surfaces with an
accuracy of better than 15 nm RMS over a 20 mm aperture, giving the designer great freedom in generating powerful
and unorthodox designs. However, the complexity of these freeform surfaces poses a challenge to their accurate
characterisation. This paper will discuss in detail the metrology of a specific freeform component in the instrument. The
form of these complex astigmatic surfaces was measured using spherical wavefronts by adapting a tilted Twyman-Green
Interferometer arrangement. There are eight separate designs for this type of component, each with a different orientation
and magnitude of astigmatism. Careful mechanical fixturing is essential to align the astigmatic axis to the test set up.
The impact of mechanical tolerances on measurement uncertainty will be discussed in detail.
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We have developed a metrology system that is capable of measuring rough ground and polished surfaces alike, has
limited sensitivity to the nominal surface shape, and can accommodate surfaces up to 8.4 m in diameter. The system
couples a commercial laser tracker with an advanced calibration technique and a system of stability references to
mitigate numerous error sources. This system was built to guide loose abrasive grinding and initial polishing of the off-axis
primary mirror segments for the Giant Magellan Telescope (GMT), and is also being used to guide the fabrication of
the Large Synoptic Survey Telescope primary and tertiary mirrors. In addition to guiding fabrication, the system also
works as a verification test for the GMT principal optical interferometric test of the polished mirror segment to
corroborate the measurement in several low-order aberrations. A quantitative assessment of the system accuracy is
presented, along with measurement results for GMT, including a comparison to the optical interferometric test of the
polished surface.8
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Aspheric surfaces, such as telescope mirrors, are commonly measured using interferometry with computer generated
hologram (CGH) null correctors. The interferometers can be made with high precision and low noise, and CGHs can
control wavefront errors to accuracy approaching 1 nm for difficult aspheric surfaces. However, such optical systems
are typically poorly suited for high performance imaging. The aspheric surface must be viewed through a CGH that was
intentionally designed to introduce many hundreds of waves of aberration. The imaging aberrations create difficulties
for the measurements by coupling both geometric and diffraction effects into the measurement. These issues are
explored here, and we show how the use of larger holograms can mitigate these effects.
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The Hobby-Eberly Telescope (HET) Wide-Field Upgrade (WFU) will be equipped with new metrology systems to
actively control the optical alignment of the new four-mirror Wide-Field Corrector (WFC) as it tracks sidereal motion
with respect to the fixed primary mirror. These systems include a tip/tilt sensor (TTS), distance measuring
interferometers (DMI), guide probes (GP), and wavefront sensors (WFS). While the TTS and DMIs are to monitor the
mechanical alignment of the WFC, the WFSs and GPs will produce direct measurement of the optical alignment of the
WFC with respect to the HET primary mirror. Together, these systems provide fully redundant alignment and pointing
information for the telescope, thereby keeping the WFC in focus and suppressing alignment-driven field aberrations.
We describe the current snapshot of these systems and discuss their roles, expected performance, and operation plans.
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Aspherical optics are widely used in modern optical telescopes and instrumentation because of their ability to
reduce aberrations with a simple optical system. Testing their optical quality through null interferometry is not
trivial as reference optics are not available. Computer-Generated Holograms (CGHs) are efficient devices that
allow to generate a well-defined optical wavefront. We developed rewritable Computer Generated Holograms
for the interferometric test of aspheres based on photochromic layers. These photochromic holograms are cost-effective
and the method of production does not need any post exposure process.
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At the Southern African Large Telescope (SALT), in collaboration with FOGALE Nanotech, we have been testing the recently-developed new generation inductive edge sensors. The Fogale inductive sensor is one
technology being evaluated as a possible replacement for the now defunct capacitance-based edge sensing system.
We present the results of exhaustive environmental testing of two variants of the inductive sensor. In addition to the environmental testing including RH and temperature cycles, the sensor was tested for sensitivity to dust and metals. We also consider long-term sensor stability, as well as that of the electronics and of the glue used to bond the sensor to its supporting structure. A prototype design for an adjustable mount is presented which will allow for in-plane gap and shear variations present in the primary mirror configuration without adversely disturbing the figure of the individual mirror segments or the measurement accuracy.
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An interferometric null Test Method is described for testing convex aspheric surfaces, such as found in secondary
mirrors of Cassegrain telescopes or variations thereof such as Mersenne and Ritchey-Chrétien. A family of test designs
is described covering a wide range of mirror diameters, radii of curvature, and aspheric shapes as described by conic
constants and/or polynomials. The Test Method has been used successfully for testing the convex hyperboloid surface
of the 244-mm diameter secondary mirror of the NASA 3-meter IRTF telescope. The Test Method is currently being
used to test the 120-mm diameter, convex paraboloid secondary mirrors of the Magdalena Ridge Observatory
Interferometer (MROI). Test designs exist on paper for both Keck secondary mirrors (0.53-m and 1.4-m diameter),
the HST secondary (0.3-meter diameter), and secondary mirrors of some of the extremely large telescopes of the
future, such as the TMT secondary (3.2-m diameter). In typical test embodiments, the simplicity of the Test enables
rapid implementation at a fraction of the cost of comparable Hindle-Sphere or Hindle-Simpson tests.
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The new giant telescopes can be compared to space projects. They will require ground-based test
support equipment to fully characterize the optical sub-system functionalities and performances
before costly commissioning on the telescope. The support equipment must be designed to reproduce
the telescope or the front optical system's aberrated wavefront. We show that the aberrated
wavefront can be generated at low cost by a magnetic liquid deformable mirror. A prototype 91-
actuator liquid deformable mirror having a diameter of 33 mm was built and used to simulate the
off-axis aberrated CFHT's primary mirror up to 0.5 degrees FOV.
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The scanning pentaprism system for testing the 8.4 m off-axis segments for the Giant Magellan Telescope has recently
been completed. The system uses a fiber source and a carriage mounted pentaprism to scan a 40 mm collimated beam
across the surface of the segment under test. Since the scanning beam is parallel to the optical axis of the parent mirror, it
comes to focus on a detector at the telescope's prime focus, where displacement of the spot is proportional to the slope
error. A second collimated beam from a stationary reference pentaprism is used to compensate for any changes in the
relative positions of the optical components during testing. The optical components are suspended over the mirror on a
rail system that can be rotated so that scans can be made across any diameter of the segment. The test is capable of
measuring wavefront slope errors to 1 μrad rms, adequate to verify that power, astigmatism, coma, and other low-order
aberrations are small enough to be corrected easily at the telescope with the segment's active support system.
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The Southern African Large Telescope (SALT) recently (2008) abandoned attempts at using capacitive mirror edge
sensors, mainly due to poor performance at a relative humidity above ~60%, a not infrequent occurrence. Different
technologies are now being explored for alternative sensors on SALT. In this paper we describe the design and
development of a novel prototype optical edge sensor, based on the application of the interferential scanning principle,
as used in optical encoders. These prototype sensors were subsequently tested at SAAO and ESO, for potential
application on SALT and E-ELT.
Environmental tests, conducted in climatic control chambers, looked at temperature and relative humidity sensitivity,
long term stability and sensor noise. The temperature sensitivity for height and gap were, respectively, 10nm/°C and
44nm/°C, while for relative humidity they were 4nm/10% and 50nm/10%, respectively. These either met, or were close
to, the SALT specification. While there were significant lags in response, this was due to the sensor's relatively large
mass (~200 gm per sensor half), which was not optimized. This is likely to improve, should a revised design be
developed in future. Impressively the sensor noise was <0.015 nm RMS, over three orders of magnitude better than the
specification. Our conclusions are that optical edge sensing is a viable technique for use on segmented mirror telescopes.
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Volume phase holographic (VPH) gratings are increasingly being used as diffractive elements in astronomical
instruments due to their potential for very high peak diffraction efficiencies and the possibility of a compact instrument
design when the gratings are used in transmission. Historically, VPH grating (VPHG) sizes have been limited by the size
of manufacturer's holographic recording optics. We report on the design, specification and fabrication of a large, 290
mm × 475 mm elliptically-shaped, mosaic VPHG for the Apache Point Observatory Galactic Evolution Experiment
(APOGEE) spectrograph. This high-resolution near-infrared multi-object spectrograph is in construction for the Sloan
Digital Sky Survey III (SDSS III). The 1008.6 lines/mm VPHG was designed for optimized performance over a
wavelength range from 1.5 to 1.7 μm. A step-and-repeat exposure method was chosen to fabricate a three-segment
mosaic on a 305 mm × 508 mm monolithic fused-silica substrate. Specification considerations imposed on the VPHG to
assure the mosaic construction will satisfy the end use requirements are discussed. Production issues and test results of
the mosaic VPHG are discussed.
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The Vector Vortex Coronagraph is a phase-based coronagraph, one of the most efficient in terms of inner working
angle, throughput, discovery space, contrast, and simplicity. Using liquid-crystal polymer technology, this new
coronagraph has recently been the subject of lab demonstrations in the near-infrared, visible and was also used
on sky at the Palomar observatory in the H and K bands (1.65 and 2.2 μm, respectively) to image the brown
dwarf companion to HR 7672, and the three extra-solar planets around HR 8799. However, despite these recent
successes, the Vector Vortex Coronagraph is, as are most coronagraphs, sensitive to the central obscuration
and secondary support structures, low-order aberrations (tip-tilt, focus, etc), bandwidth (chromaticism), and
polarization when image-plane wavefront sensing is performed. Here, we consider in detail these sensitivities as
a function of the topological charge of the vortex and design features inherent to the manufacturing technology,
and show that in practice all of them can be mitigated to meet specific needs.
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MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment) will be a mid-infrared spectro-interferometer
combining the beams of up to four telescopes of the European Southern Observatory Very Large Telescope
Interferometer (ESO VLTI), providing phase closure and image reconstruction. Matisse will produce interferometric
spectra in the LM and in the N band (2.3 to 13.5 micron) and is as such a successor of MIDI. The instrument will be
developed by a consortium consisting of Observatoire de Nice (warm optics), NOVA-ASTRON (cold optics), MPI-A
(cryostats) and MPIfR (detectors).
Beams of up to four Unit Telescopes or Auxiliary Telescopes (UT - AT) pass the warm pre-optics and in the cold optics
all beams recombine on the detector where they create a spectral interference pattern.
An innovative MAIT plan drastically shortens the MAIT phase and therefore reduces cost. The MAIT plan comprises the
assembly and alignment procedure of about 220 cryogenic optical components for which a mirror mount clip has been
developed. Alignment accuracy and stability specifications are of the order of nanometers and arcsec, which requires
over 50 degrees of freedom in cryogenic alignment mechanisms for e.g. Tip/Tilt and detector Tip/Tilt/Focus. The design,
realization and test results of these mechanisms are presented. A cryogenic electrical switch significantly reduces the
complexity of the electronic cabling and improves reliability.
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We discuss the Clover cryostats, which are dry cryostats containing three stages of cooling; a pulse tube cooler,
a sorption fridge and a continuous miniature dilution refrigerator. We describe the thermal architecture of the
system and present thermal data for the various stages including its performance when tilted.
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The Near Infrared Spectrograph (NIRSpec) developed by EADS Astrium GmbH for the European Space Agency (ESA)
is a spectrograph covering the 0.6-5.0 μm waveband to fly on the James Webb Space Telescope (JWST). NIRSpec will
be primarily operated as a multi-object spectrograph but also includes an integral field unit (IFU) allowing a 3×3 arcsec
field of view to be sampled continuously with 0.1 arcsec spatial resolution. The IFU, based on an advanced image slicer
concept, is a very compact athermal unit made of aluminium. It contains three 30-element monolithic mirror arrays
forming slicer, pupil and slit mirrors, and single-surface image relay and plane fold mirrors, produced using 5-axis
diamond-machining techniques. Many of the mirrors have complex surfaces like toric sections with 3rd-order corrections
in order to achieve the required performance within a small allowed volume, and could only have been fabricated with
the most advanced free-form machining. The mechanical design accommodates the differential expansion between the
aluminium IFU and its titanium parent assembly across a 250K drop to operating temperature using an isostatic
mounting system. This paper presents the development of the IFU from the design and diamond-machining techniques
to the optical and cryogenic testing of the assembled flight model unit.
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The Mid Infrared Instrument (MIRI) aboard JWST is equipped with one filter wheel and two dichroic-grating wheel
mechanisms to reconfigure the instrument between observing modes such as broad/narrow-band imaging, coronagraphy
and low/medium resolution spectroscopy. Key requirements for the three mechanisms with up to 18 optical elements on
the wheel include: (1) reliable operation at T = 7 K, (2) high positional accuracy of 4 arcsec, (3) low power dissipation,
(4) high vibration capability, (5) functionality at 7 K < T < 300 K and (6) long lifetime (5-10 years). To meet these
requirements a space-proven wheel concept consisting of a central MoS2-lubricated integrated ball bearing, a central
torque motor for actuation, a ratchet system with monolithic CuBe flexural pivots for precise and powerless positioning
and a magnetoresistive position sensor has been implemented. We report here the final performance and lessons-learnt
from the successful acceptance test program of the MIRI wheel mechanism flight models. The mechanisms have been
meanwhile integrated into the flight model of the MIRI instrument, ready for launch in 2014 by an Ariane 5 rocket.
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Infrared radiometers and spectrometers generally use blackbodies for calibration, and with the high accuracy needs of
upcoming missions, blackbodies capable of meeting strict accuracy requirements are needed. One such mission, the
NASA climate science mission Climate Absolute Radiance and Refractivity Observatory (CLARREO), which will
measure Earth's emitted spectral radiance from orbit, has an absolute accuracy requirement of 0.1 K (3σ) at 220 K over
most of the thermal infrared. Space Dynamics Laboratory (SDL) has a blackbody design capable of meeting strict
modern accuracy requirements. This design is relatively simple to build, was developed for use on the ground or onorbit,
and is readily scalable for aperture size and required performance. These-high accuracy blackbodies are currently
in use as a ground calibration unit and with a high-altitude balloon instrument. SDL is currently building a prototype
blackbody to demonstrate the ability to achieve very high accuracy, and we expect it to have emissivity of ~0.9999 from
1.5 to 50 μm, temperature uncertainties of ~25 mK, and radiance uncertainties of ~10 mK due to temperature gradients.
The high emissivity and low thermal gradient uncertainties are achieved through cavity design, while the low
temperature uncertainty is attained by including phase change materials such as mercury, gallium, and water in the
blackbody. Blackbody temperature sensors are calibrated at the melt points of these materials, which are determined by
heating through their melt point. This allows absolute temperature calibration traceable to the SI temperature scale.
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The Euclid mission is currently being developed within the European Space Agency's Cosmic Vision Program.
The five year mission will survey the entire extragalactic sky (~ 20 000 deg2) with the aim of constraining the
nature of dark energy and dark matter. The spacecraft's payload consists of two instruments: one imaging
instrument, which has both a visible and a near-infrared channel, and one spectroscopic instrument operating in
the near-infrared wavelength regime. The two channels of the imaging instrument, the Visible Imaging Channel
(VIS) and the Near-Infrared Imaging Photometer Channel (NIP), will focus on the weak lensing science probe.
The large survey area and the need to not only image each patch of sky in multiple bands, but also in multiple
dithers, requires over 640 000 operations of the NIP channel's filter wheel mechanism. With a 127 mm diameter
and a mass of ~ 330 g per element, these brittle infrared filters dictate highly demanding requirements on this
single-point-failure mechanism. To accommodate the large filters the wheel must have an outer diameter of
~ 400 mm, which will result in significant loads being applied to the bearing assembly during launch.
The centrally driven titanium filter wheel will house the infrared filters in specially designed mounts. Both
stepper motor and brushless DC drive systems are being considered and tested for this mechanism. This paper
presents the design considerations and details the first prototyping campaign of this mechanism. The design and
finite element analysis of the filter mounting concept are also presented.
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TNO, together with its partners Micromega and SRON, have designed a cryogenic scanning mechanism for use in the
SAFARI Fourier Transform Spectrometer (FTS) on board of the SPICA mission.
The optics of the FTS scanning mechanism (FTSM) consists of two back-to-back cat's-eyes. The optics are mounted on a
central "back-bone" tube which houses all the important mechatronic parts: the magnetic bearing linear guiding system, a
magnetic linear motor serving as the OPD actuator, internal metrology with nanometer resolution, and a launch lock.
A magnetic bearing is employed to enable a large scanning stroke in a small volume. It supports the optics in a free-floating
way with no friction, or other non-linearities, enabling sub-nanometer accuracy within a single stage with a
stroke of -4 mm to +31.5 mm.
Because the FTSM will be used at cryogenic temperatures of 4 Kelvin, the main structure and optics are all constructed
from 6061 Aluminum. The overall outside dimensions of the FTSM are: 393 x 130 x 125 mm, and the mass is 2.2 kg.
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The Sardinia Radio Telescope (SRT) is a 64 meters (diameter) single dish radioantenna which is in the building phase in
Italy. One of the most challenging characteristics of SRT is its capability to observe up to a frequency of 100 GHz thanks
to its main reflector active surface. The active surface is composed by 1008 panels and 1116 mechanical actuators which
may modify the segmented shape of the main reflector making possible the correction for wavefront distortions induced
by the gravitational and thermal deformations.
In order to observe at a frequency of 100 GHz the surface shape must be accurate below of a value of 150 μm r.m.s..
This value may be reached during the initial alignement phase using the microwave holography but it cannot be
maintained during the scientific operations because of the (dynamical) deformations. In order to permit the observations
at any time, a system able to measure the surface deformations with the necessary accuracy and a time-response of few
minutes (the time-scale of the deformations) must be operative.
We propose here three simple and robust methods to measure the relative deformations of the segmented panels with
respect to an initial aligned surface (reference surface). The ultimate choice on which one of the three systems will
operate on SRT will be taken after final testing on all of them. Prototypes of each system have been realized and two of
them have been also successfully tested on the active optics radiotelescope of Noto (Italy). The test on the third system
will be done in the next few months.
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The complexity and size of instruments for next generation telescopes demands innovative approaches to existing
problems. Within this framework, we present MAPS; a Micro Autonomous Positioning System for mirror deployment in
an E-ELT instrument such as EAGLE. The micro-robots have a 25mmx25mm footprint and utilise RF communications
and small rechargeable batteries to be completely wireless. Coarse positioning and fine alignment is achieved through
the use of miniature gear motors and gearheads. Positional information is determined externally and corrective motions
relayed to the robots. This paper reports on the challenges which such a system presents, current developments, and areas
of expected future research.
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We report on the technological achievements of our latest Starbug prototypes and their implications for smart focal plane
fiber positioning applications for wide-field astronomy. The Starbugs are innovative self-motile miniature robotic
devices that can simultaneously and independently position fibers or payloads over a field plate located at the telescope's
focal plane. The Starbugs concept overcomes many of the limitations associated with the traditional 'pick and place'
positioners where a robot places fixed buttons onto the field plate. The new Starbug prototypes use piezoelectric
actuators and have the following features: (i) new 'lift-and-step' method (discrete step) for accurate positioning over
different surfaces; and (ii) operate in an inverted hanging position underneath a transparent field plate, removing the need
for fibercable retractors. In this paper, we present aspects of the Starbug prototypes, including the theoretical model,
mechanical design, experimental setup, algorithms, performance and applications for astronomical instrumentation.
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The two antenna transporters of the ALMA Observatory are used to relocate 12-meter antennas and to move them
between low (3000m) and high altitude site (5000m). When analyzing the dynamic response of the transporters to road
ripple of critical height and inter-distance, it turned out that transporter accelerations exceeding the seismic accelerations
cannot be excluded. This is considered as a remote risk however with possibly catastrophic consequences for the
equipped antennas. The problem was analyzed by ESO experts for dynamic simulations and an additional damping
system was designed to limit the loads to acceptable values.
This paper describes the design of the additional damping system including its concept, the model-based design using
dynamic simulations and verification tests on site.
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Generally, panels of radio telescopes are mainly shaped in trapezoid and each is supported/positioned by four adjustors
beneath its vertexes. Such configuration of panel supporting system is essentially hyper-static, and the panel is overconstrained
from a kinematic point of view. When the panel is to be adjusted and/or actuated, it will suffer stress from its
adjusters and hence its shape is to be distorted. This situation is not desirable for high precision panels, such as glass
based panels especially used for sub-millimeter and shorter wavelength telescopes with active optics/active panel
technology. This paper began with a general overview of panel patterns and panel supports of existing radio telescopes.
Thereby, we proposed a preferable master-slave active surface concept for triangular and/or hexagonal panel pattern. In
addition, we carry out panel error sensitivity analysis for all the 6 degrees of freedom (DOF) of a panel to identify what
DOFs are most sensitive for an active surface. And afterwards, based on the error sensitivity analysis, we suggested an
innovative parallel-series concept hexapod well fitted for an active panel to correct for all of its 6 rigid errors. A
demonstration active surface using the master-slave concept and the hexapod manifested a great save in cost, where only
486 precision actuators are needed for 438 panels, which is 37% of those actuators needed by classic segmented mirror
active optics. Further, we put forward a swaying-arm based design concept for the related connecting joints between
panels, which ensures that all the panels attached on to it free from over-constraints when they are positioned and/or
actuated. Principle and performance of the swaying-arm connecting mechanism are elaborated before a practical cablemesh
based prototype active surface is presented with comprehensive finite element analysis and simulation.
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After suffering from serious problems in the course of the SiC 1.5m M1 manufacturing, the existing design of the M1,
it's cell and the associated mirror cooling system was investigated in terms of modification efforts to be compatible for a
different M1 substrate (Zerodur). The analysis included the system requirements, the M1 design, the M1 support system,
the M1 cooling system as well as the M1 cell.
The investigations resulting in a modified design of the above mentioned system. Driven by the choice of material,
different requirements became design driving factors. The consequences on the detail design of the M1 Mirror as well as
on the support system and the cooling system are presented.
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The Large Synoptic Survey Telescope (LSST) utilizes an 8.4-meter cast borosilicate primary/tertiary mirror (M1M3).
This mirror system has stringent vibration and stiffness requirements because the LSST optical system does not include a
fast steering mirror and the mission requires a short slew and settling time. The position stability of the M1M3 relative to
the mirror cell is controlled by six displacement controlled actuators (subsequently referred to as "hardpoints") that form
a large hexapod. This design is based largely on previous hardpoints implemented for borosilicate mirror positioning.
Traditionally, all dynamic forces applied to these mirrors are reacted through their hardpoints. Consequently, the
characteristics of these hardpoints critically affect the ability of the telescope to meet the stringent dynamic requirements
without overstressing the mirror. The hardpoints must have a high stiffness of 120 N/um in the axial direction, while
protecting the mirror by limiting the loads in all six degrees of freedom. The non-axial direction loads are limited by
flexures. The axial loads are limited by a pneumatic breakaway mechanism. Since the hardpoints react the dynamic
mirror loads, the axial breakaway force may limit the telescope's slewing accelerations. The travel of the breakaway
mechanism must accommodate the transfer of the mirror from its active supports to its static supports. The hardpoint
positioning mechanism must have sufficient travel and resolution to properly position the mirror relative to the mirror
cell. Fulfilling these functions also requires numerous sensors, including a precision axial load cell which is paramount
in determining the figure control actuator forces.
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The drive and bearing technologies have a major impact on the static and dynamic performance of steerable
structures such as telescope and dome. Merging drive and bearing system into friction drive mechanical devices
(bogie) can reduce the complexity and cost of the design. In the framework of ELT design study (European
FP6) a breadboard test setup was realized to test and evaluate the static and dynamic behavior of such bogies.
In this paper some of the characterization test results are presented. Characterization of the bogies and the
setup structure in the frequency domain, quantification and measure of the most important parameters of the
friction forces, the control of the bogies and the tracking performance of the test setup are among the main
results discussed in this paper.
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The Magdalena Ridge Interferometer (MROI) is a project which comprises an optical array of up to ten relocatable 1.4m
telescopes arranged in a "Y" configuration. Each of these telescopes will be housed inside a Unit Telescope Enclosure
(UTE) which can be lifted and moved onto any of 28 stations. This paper presents a general description of how the
constraints imposed by the requirements for the close-pack configuration and relocatability led to the design of an
innovative, compact and light-weight enclosure of small diameter and high structural strength. The unique internal layout
gives sufficient space inside to house, not only to house the telescope mount, but also associated electronics, nasmyth
table opto-mechanical equipment and beam relay system interface.
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Deformable mirrors actuated by smart structures are promising devices for next generation astronomical instrumentation.
Thermal activated Shape Memory Alloys are materials able to recover their original shape, after an
external deformation, if heated above a characteristic temperature. If the recovery of the shape is completely
or partially prevented by the presence of constraints, the material can generate recovery stress. Thanks to this
feature, these materials can be positively exploited in Smart Structures if properly embedded into host materials.
This paper will show the technological processes developed for an efficient use of SMA-based actuators embedded
in smart structures tailored to astronomical instrumentation. In particular the analysis of the interface with the
host material. Some possible modeling approaches to the actuators behavior will be addressed taking into account
trade-offs between detailed analysis and overall performance prediction as a function of the computational
time. We developed a combined Finite Element and Raytracing analysis devoted to a parametric performance
predictions of a SMA based substrate applicable to deformable mirrors. We took in detail into account the possibility
to change the focal length of the mirror keeping a satisfactory image quality. Finally a possible approach
with some preliminary results for an efficient control system for the strongly non-linear SMA actuators will be
presented.
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Deformable mirrors actuated by smart structures are promising devices for next generation astronomical instrumentation.
The piezo technology and in particular piezoceramics is currently among the most investigated
structural materials. Fragility makes Ceramic materials extremely vulnerable to accidental breakage during bonding
and embedding processes and limits the ability to comply to curved surfaces (typical of mirrors). Moreover
lead-based piezoceramics typically have relevant additional masses. To overcome these limitations, we studied
the applicability of composites piezoceramics actuators to smart structures with these purposes. We developed
a combined Finite Element and Raytracing analysis devoted to a parametric performance predictions of a smart
Piezocomposites based substrate applicable to deformable mirrors. We took in detail into account the possibility
to change the focal length of the mirror keeping a satisfactory image quality. In this paper we present a specific
type of Piezocomposite actuators and numerical/experimental techniques purposely developed to integrate them
into smart structures. We evaluated numerical and experimental results comparing bonding and embedding of
these devices.
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The solar telescope EST is currently in the conceptual design phase. It is planned to be build on the Canary Islands until
end of the decade. It is specialized on polarimetric observations and will provide high spatial and spectral observations of
the different solar atmospheric layers.
The diameter of the primary mirror blank is 4.2m. Different types of mirror shapes were investigated with respect to
thermal and mechanical characteristics.
To remove the absorbed heat an air cooling system from the back side will be applied. Additional an air flushing system
will remove remaining warm air from the front side.
A major problem of a large open telescope will be the wind load. Results of the investigations will be shown. To achieve
optimal optical performance an active support system is planned. The primary mirror cell needs to be stiff enough to
support the primary mirror without deformation at strong wind in case of the open telescope option, but sufficient room
for the active support system and cooling system below the backside of the mirror is also required. Preliminary designs
and analysis results will be presented.
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The Large Synoptic Survey Telescope (LSST) flat-fields must repeatedly trace not only the spatial response variations,
but also the chromatic response through the entire optical system, with an accuracy driven by the photometric
requirements for the LSST survey data. This places challenging requirements on the LSST Calibration Dome Screen,
which must uniformly illuminate the 8.4-meter diameter telescope pupil over its 3.5-degree field of view at desired
monochromatic wavelengths in a way that allows the measurement of the total system throughput from entrance pupil to
the digitization of charge in the camera electronics. This includes the reflectivity of the mirrors, transmission of the
refractive optics and filters, the quantum efficiency of the sensors in the camera, and the gain and linearity of the sensor
read-out electronics. The baseline design uses a single tunable laser and includes an array of discrete projectors. The
projected flux of light produced by the screen must fill the entire telescope pupil and provide uniform illumination to 1%
at the focal plane and to within 0.25% over any optical trajectory within 0.5 degrees of each other. The wavelength of
light is tunable across the LSST bandpass from 320 nm to 1080 nm. The screen also includes a broad-band ("white")
light source with known Spectral Energy Density (SED) that spans the same range of wavelengths.
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Design-to-cost exercises and innovative design have resulted in remarkably
high performing half-meter class wide field astronomical telescopes. This
approach is being extended to meter+ class telescopes with further innovation
on mounts and optics. Custom motors, drives and bearings have been
developed to keep performance up and cost down. We will also report on a
concurrent engineering campaign with Brashear Optics to ensure optical
performance while maintaining the highest value for primary mirrors in our line
of meter (and larger) class astronomical telescopes.
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The performance of the C-ring telescope mount rivals other designs in stiffness, tracking, simplicity, lack of field
rotation, mechanical size and operating envelope. Issues relating to cost, fabrication, and complexity have suppressed
the prevalence of the C-ring mount. The Las Cumbres Observatory Global Telescope (LCOGT) robotic C-ring telescope
mounts, built for its network of 1.0m and 0.4m telescopes, solve many of these issues. The design yields a scalable
mount with performance capabilities well suited for telescopes located at the best astronomical sites in the world at a low
cost. Pointing has been demonstrated to be under 7 arc-sec RMS. Unguided tracking performance is 0.6 arc-sec for 1
minute and 2 arc-sec for 15 minutes. Slew speeds of 10deg/sec are reliably used with sub-second settling times. The
mount coupled with the 1.0m telescope yields a well damped 16 Hz system. Axes are driven with zero backlash direct
drive motors with a 0.01 arc-sec resolution. High system bandwidth yields superb disturbance rejection making it ideal
for open air operation. Drive and bearings are maintenance free and feature a novel "bug cover" to seal them from wear
and damage. Low costs are achieved with the drive/feedback configuration, structure design, and fabrication techniques,
as well minimizing operating and maintenance.
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Scientific performance specifications, a necessity for ease of commissioning and minimal maintenance, and a desire for
automated sensing and remote collimation have led to novel designs and features in LCOGT's one-meter Optical Tube
Assembly (OTA). We discuss the design and performance of the quasi-RC optical system with 18 point whiffletree and
radial hub mount. Position probes and IR temperature sensors on the primary and secondary mirrors give feedback for
active collimation and thermal control. A carbon fiber/epoxy composite truss, with unique spherical node connections,
mounts to parallel and offset Invar vanes. A flexure based, closed loop, 3-DOF secondary mirror mechanism is used for
tip/tilt collimation. The optics and deflections of the OTA components were iteratively designed for passive collimation
with a changing gravity vector. We present the FEA predictions, measured deflections, and measured hysteresis for
many of the components. Vibration modes, amplitudes, and damping of the system are presented with an FFT frequency
analysis. Thermal CTE effects on loading and focal position are quantified. All of these system effects are then related to
the overall scientific performance of the 1.0 m telescope.
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The Discovery Channel Telescope (DCT) is a 4.3-meter telescope designed for dual optical configurations, featuring an
f/6.1 Ritchey-Chretien prescription with a 0.5° field-of-view, and a corrected f/2.3 prime focus with a 2° field-of-view.
The DCT Active Optics System (AOS) maintains collimation and mirror figure to provide seeing limited images across
the focal planes and rapid settling times to minimize observing overhead, using a combination of feed-forward and lowbandwidth
feedback control via wavefront sensing. Collimation is maintained by tip-tilt-piston control of the M2
assembly and articulating M1 within its cell, taking advantage of the 120 degree-of-freedom support used for figure
control. We present an overview of the AOS design and principles of operation, and a summary of progress and results
to date.
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The hexapod is a parallel kinematic manipulator that is the minimum arrangement for independent control of six
degrees of freedom. Advancing needs for hexapod performance, capacity and configurations have driven
development of highly capable new actuator designs. This paper describes new compact hexapod design proposals
for high load capacity, and corresponding hexapod actuator only mechanisms suitable for integration as structural motion
elements in next-generation telescope designs. These actuators provide up to 90 000N load capability while preserving
sub-micrometer positional capability and in-position stability. The design is optimized for low power dissipation and
incorporates novel encoders direct manufactured with the nut flange to achieve more than 100000 increments per
revolution. In the hexapod design we choose cardan joints for the actuator that have axis offsets to provide
optimized stiffness. The additional computational requirements for offset axes are readily solved by advanced
kinematic algorithms and modern hardware. The paper also describes the hexapod controller concept with
individual actuator designs, which allows the integration of hexapod actuators into the main telescope structure to
reduce mass and provide the telescope designer more design freedom in the incorporation of these types of motion
systems. An adaptive software package was developed including collision control feature for real-time safety during
hexapod movements.
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The close-pack array of the MROI necessitated an original design for the Unit Telescope Enclosure (UTE) at Magdalena Ridge Observatory. The Magdalena Ridge Observatory Interferometer (MROI) is a project which comprises an array of up to ten (10) 1.4m diameter mirror telescopes arranged in a "Y" configuration. Each of these telescopes will be housed inside a Unit Telescope Enclosure (UTE) which are relocatable onto any of 28 stations. The most compact configuration includes all ten telescopes, several of which are at a relative distance of less than 8m center to center from each other. Since the minimum angle of the field of regard is 30° with respect to the horizon, it is difficult to prevent optical blockage caused by adjacent UTEs in this compact array.
This paper presents the design constraints inherent in meeting the requirement for the close-pack array. An innovative design enclosure was created which incorporates an unique dome/observing aperture system. The description of this system focuses on how the field of regard requirement led to an unique and highly innovative concept that had to be able to operate in the harsh environmental conditions encountered at an altitude of 10,460ft (3,188m).
Finally, we describe the wide use of composites materials and structures (e.g. glass/carbon fibres, sandwich panels etc.) on the aperture system which represents the only way to guarantee adequate thermal and environmental protection, compactness, structural stability and limited power consumption due to reduced mass.
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Astronomical optics are often exposed to moisture and dust in observatory environments, which frequently compromises
their high-performance coatings. Suitable protective layers to resist dust and moisture accumulation would be extremely
advantageous, but have received scant attention thus far. Hydrophobic and scratch-resistant coatings, developed
primarily for opthalmic use, exhibit several attractive properties for astronomical optics. We examine the properties of
one such coating and its applicability to astronomical mirrors and lenses. This includes efficiency of dust removal,
abrasion resistance, moisture resistance, ease of stripping, and transmission across a wide wavelength range.
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Rugate designs for the realization of notch filters are well known in the literature. The required deposition of gradient
index layers is difficult to manufacture. In our approach we apply the equivalent index theory to replace the gradient
index profile of a notch filter design. We produce single and multiple notch filters with plasma ion-assisted deposition
and broad-band optical monitoring. As examples, a 500nm notch filter for the GREGOR telescope and a 589nm notch
filter for the GALACSI instrument of the VLT are discussed. Additionally, a 4-line multiple notch filter and a 218nm
notch filter made for fluorescence spectroscopy applications are presented.
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Context. Dark fringe interferometry in the thermal infrared is one way to detect directly a planet orbiting a star, and so to
characterize the planet's atmosphere through spectroscopy. This method demands a phase shift of π1 in one arm of the
interferometer. In order to detect various bio-tracers gases, a broad wavelength range (6-18 μm)2-3 is necessary, therefore
an achromatic phase shift of π is required. The achromatic device presented here is a phase shifter made of two cellular
mirrors, in which each cell induces a specific phase shift.
Aims. We wish to demonstrate that this theoretical concept is experimentally valid. We present in this paper the setup
and the very first results.
Methods. In a first step, we have consolidated the theoretical ground and in a second step we developed an optical bench
in the visible domain to test the concept and measure the performances of this device.
Results. The preliminary experimental tests show evidences that such a device is working as expected in terms of nulling
and achromatism: in spite of an error on one cell of the prototype, it provides a nulling of 2.10-3 at one wavelength, and
this value is close to the expected value. Besides, a nulling of 1.10-2 in a 450 to 750 nm bandwidth: a hint that a perfect
device should be achromatic.
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Stellar coronagraphy is a key technology for current and future instruments for exoplanet imaging and spectroscopy,
both on the ground and in space. We pursue the research on coronagraphs based on circular phase masks
and report in this paper on recent advances in terms of the trade between spectral bandwidth and
achievable contrast. Circular phase masks combined with colored apodizations prove to be promising options in
such coronagraphic systems to reach high contrast gains within the search area over a wide band of wavelengths.
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We are developing high performance mid-infrared (especially 30-40μm wavelength regions) multilayer interference
filters with mechanical strength and robustness for thermal cycling toward cryogenic infrared astronomical missions.
Multilayer interference filters enable us to design a wide variety of spectral response by controlling refractive index and
thickness of each layer. However, in mid- and far-infrared (MIR/FIR) regions, there are a few optical materials so that
we can only use limited refractive index values to design filters, which makes difficult to realize high performance
filters. It is also difficult to deposit thick layers required for MIR/FIR multilayer filters. Furthermore, deposition of two
materials, which have different coefficients of thermal expansion, makes filters fragile for thermal cycling. To clear these
problems, we introduce sub-wavelength structures (SWS) for controlling the refractive index. Then, only one material is
necessary for fabricating filters, which enables us to fabricate filters with mechanical strength and robustness for thermal
cycling. According to the effective medium approximation (EMA) theory, the refractive index of randomly mixing
materials in sub-wavelength scale is controllable by changing the ratio of mixing materials. However, it is not clear that
EMA can be applied to such simple SWS, periodic cylindrical holes on a bulk material, which is easily fabricated by
photolithography. In order to verify the controllability of refractive index by simple SWS, we have fabricated simple
SWS on a silicon substrate and measured its transmittance. Comparing measured transmittance with theoretical
transmittance calculated by EMA, we confirm that EMA can be applied to simple SWS fabricated by photolithography.
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Black and spectrally selective surfaces are important in optical systems. The proper selection of these surfaces is
essential to create or maintain system performance. A critical first step in selection of surfaces is to understand the
performance requirements for contrast and stray light. A well-defined performance specification accompanied by stray
light modeling is important to understand how the system behaves. Without both of these, the selection of surfaces is
very difficult. Practical considerations in choosing spectrally selective and tailored emissivity surfaces for a range of
ultraviolet/optical/infrared telescopes and instruments are given. The Bidirectional Reflectance Data Function (BRDF) of
a surface is the most useful characterization in assessing the optical properties of surfaces. Data on long-term surface
durability characteristics necessary for end of life optical predictions are also critical.
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The Key Technology Network (KTN) within the OPTICON programme has been developing a roadmap for the
technology needed to meet the challenges of optical and infrared astronomy over the next few years, with particular
emphasis on the requirements of Extremely Large Telescopes. The process and methodology so far will be described,
along with the most recent roadmap.
The roadmap shows the expected progression of ground-based astronomy facilities and the technological developments
which will be required to realise these new facilities. The roadmap highlights the key stages in the development of these
technologies.
In some areas, such as conventional optics, gradual developments in areas such as light-weighting of optics will slowly
be adopted into future instruments. In other areas, such as large area IR detectors, more rapid progress can be expected as
new processing techniques allow larger and faster arrays. Finally, other areas such as integrated photonics have the
potential to revolutionise astronomical instrumentation.
Future plans are outlined, in particular our intention to look at longer term development and disruptive technologies.
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In this publication we present the results of a detailed study into directly written multimode waveguides for astronomical
applications. We show that waveguides up to 100 ìm across can be inscribed with the cumulative heating form of this
technique. The waveguides have 2 concentric guiding regions which are elliptical; a core that has an average ellipticity
of 1.1±0.1 and an outer cladding with an ellipticity of 0.15±0.03. It was demonstrated that the ellipticity of the
waveguides could be reduced by creating "structured" waveguides which consist of several waveguides stacked together.
The 7 mm long waveguides demonstrated insertion losses at 800 nm as low as 39% when light was launched and
collected by a standard multimode fibre (50 ìm core diameter and numerical aperture of 0.12), which is representative of
the fibres currently used on astronomical installations. More importantly, we show for the first time that structured
waveguides designed to have outer cladding regions which match the dimensions of the core of the launch and collection
fibers, have lower insertion losses than structured waveguides designed to have matching core dimensions. It is believed
that by moving to longer wavelengths of operation and exploring other structuring and beam shaping techniques it may
be possible to reduce the losses even further and make these waveguides of practical use for astronomy.
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Fibre modal noise occurs in high spectral resolution, high signal-to-noise applications.
It imposes fundamental limits upon the photometric accuracy of state-of-the-art fibre-spectrograph
systems. In order to maximize the performance of current and future instruments
it is therefore essential to predict fibre modal noise. Explicitly theoretical approaches
are often restricted to specific cases, therefore this paper focuses on the conditions relevant
to astronomy. The goal of this work is to derive a reliable model which can be used for the
optimization of future spectrograph designs. We give a description of experimental investigations
undertaken in Durham, displaying preliminary results. The first laboratory tests
of a square fibre have shown modal noise characteristics that are interestingly similar to
standard circular fibre.
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Astrophotonics offers a solution to some of the problems of building instruments for the next generation of telescopes
through the use of photonic devices to miniaturise and simplify instruments. It has already proved its worth in
interferometry over the last decade and is now being applied to nightsky background suppression. Astrophotonics offers
a radically different approach to highly-multiplexed spectroscopy to the benefit of galaxy surveys such as are required to
determine the evolution of the cosmic equation of state. The Astrophotonica Europa partnership funded by the EU via
OPTICON is undertaking a wide-ranging survey of the technological opportunities and their applicability to high-priority
astrophysical goals of the next generation of observatories. Here we summarise some of the conclusions.
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Supercontinuum white light sources (SCLS) are intense, spatially coherent laser sources with a very broad and flat
spectral energy distribution which have very quickly found ubiquitous use in optical laboratories. As photonics is now
providing more and more applications for astronomical instrumentation, the possible use of SCLS as a calibration light
source for spectroscopy has been tested. A standard industrial SCLS was coupled to the calibration unit of the PMAS
integral field spectrophotometer and compared directly to the PMAS standard tungsten filament lamp that is normally
used for calibration exposures. We report on comparative measurements concerning flux, spectral energy distribution,
and temporal stability.
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Diverse field spectroscopy is a new concept in which any part of a field can be optically captured and send to the
entrance slit of a spectrograph. It is more general than integral field spectroscopy, multi-object spectroscopy and even
multi-integral-field spectroscopy which combine the two as in the KMOS instrument. In diverse field spectroscopy, point
sources and extended sources are simultaneously optically captured in an optimal way that fully use the spectrograph for
only the regions of interest; as opposed to multi-integral-field spectroscopy where rectangular or square fields are fully
captured, the capturing mechanism will follow the complex shapes of the sources removing any useless field which can
then be use for other sources instead or permit to observe larger sources. Optical switches can be programmed to transmit
any subset of the spatial elements of a field to the spectrograph. We present the different optical designs of switches that
we made, some using micromirrors arrays, others small lenses. We also present conceptual designs of low cost projects
for Échelle spectrographs as the SALT HRS and for the FMOS spectrographs on SUBARU. A critical aspect of the
designs is to minimize the cost so that the switches can be mass-produced while maintaining high optical performances.
A general discussion will be made of the relation between the total cost of the switch system plus spectrograph and the
multiplex advantage with respect to an integral-field spectrograph giving the same performances.
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The wavefront reconstruction accuracy in Shack Hartmann sensor based adaptive optics system depends on
accurate centroiding and phase estimation from measured slope values. Monte Carlo simulations of vector
matrix multiply and Fourier phase estimation methods show fluctuations in the value of wavefront reconstruction
accuracy leading to inconsistency. In this paper, it is shown that these fluctuations can be minimized and high
wavefront reconstruction accuracy can be maintained consistently by applying a dither signal on the Shack
Hartmann lenslet array. The information of the dither signal to be applied can be obtained from the wavefronts
of the past.
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Earth's atmosphere represents a turbulent, turbid refractive element for every ground-based telescope. We describe the
significantly enhanced and optimized operation of observatories supported by the combination of a lidar and
spectrophotometer that allows accurate, provable measurement of and correction for direction-, wavelength- and timedependent
astronomical extinction. The data provided by this instrument suite enables atmospheric extinction correction
leading to "sub-1%" imaging photometric precision, and attaining the fundamental photon noise limit. In addition, this
facility-class instrument suite provides quantitative atmospheric data over the dome of the sky that allows robust realtime
decision-making about the photometric quality of a night, enabling more efficient queue-based, service, and
observer-determined telescope utilization. With operational certainty, marginal photometric time can be redirected to
other programs, allowing useful data to be acquired. Significantly enhanced utility and efficiency in the operation of
telescopes result in improved benefit-to-cost for ground-based observatories.
We propose that this level of decision-making will make large-area imaging photometric surveys, such as Pan-STARRS
and the future LSST both more effective in terms of photometry and in the use of telescopes generally. The atmospheric
data will indicate when angular or temporal changes in atmospheric transmission could have significant effect across the
rather wide fields-of-view of these telescopes.
We further propose that implementation of this type of instrument suite for direct measurement of Earth's atmosphere
will enable observing programs complementary to those currently requiring space-based observations to achieve the
required measurement precision, such as ground-based versions of the Kepler Survey or the Joint Dark Energy Mission.
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The OPTIMOS-EVE instrument proposed for the E-ELT aims to use the maximum field of view available to the E-ELT
in the limit of natural or ground-layer-corrected seeing for high multiplex fibre-fed multi-object spectroscopy in the
visible and near-IR. At the bare nasmyth focus of the telescope, this field corresponds to a focal plane 2.3m in diameter,
with a plate-scale of ~3mm/arcsec. The required positioning accuracy that is implied by seeing limited performance at
this plate-scale brings the system into the range of performances of commercial off-the-shelf robots that are commonly
used in industrial manufacturing processes. The cost-benefits that may be realized through such an approach must be
offset against the robot performance, and the ease with which a useful software system can be implemented. We
therefore investigate whether the use of such a system is indeed feasible for OPTIMOS-EVE, and the possibilities of
extending this approach to other instruments that are currently in the planning stage.
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Temporal spectral astronomy or time resolved astronomy is the study of astrophysical phenomena that show
spectral variability on very short timescales. These timescales are often too short to be resolved by current
astronomical equipment. The lack of detailed observations in this area keeps important theoretical descriptions
of astronomical events unclear or incomplete. To resolve this, instruments with very high spectral resolution
and fast read-out speeds are needed. Photonic devices such as fibre Bragg gratings (FBGs) offer potential
advantages. The use of Bragg gratings in optical fibres allows for very high spectral resolution and the stability
and precision needed for the observation of the fast variation of one particular spectral line, with the potential to
observe multiple spectral lines at once. Here, we present the concept for a fibre Bragg grating based instrument
specifically for temporal spectral astronomy and we discuss the different profiles of FBGs for such applications.
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The 8-meter mirror production capacity at the University of Arizona is well known. As the Arizona Stadium facility is
occupied with giant mirrors, we have developed capability for grinding, polishing, and testing 4-m mirrors in the large
optics shop in the College of Optical Sciences. Several outstanding capabilities for optics up to 4.3 meters in diameter
are in place:
A 4.3-m computer controlled grinding and polishing machine allows efficient figuring of steeply aspheric and nonaxisymmetric
surfaces.
Interferometry (IR and visible wavelengths) and surface profilometry making novel use of a laser tracker allows quick,
accurate in-process measurements from a movable platform on a 30-m vertical tower.
A 2-meter class flat measured with a 1-m vibration insensitive Fizeau interferometer and scanning pentaprism system;
stitching of 1-m sub-apertures provides complete surface data with the technology ready for extension to the 4 m level.
These methods were proven successful by completion of several optics including the 4.3-m Discovery Channel
Telescope primary mirror. The 10 cm thick ULE substrate was ground and polished to 16 nm rms accuracy,
corresponding to 80% encircled energy in 0.073 arc-second, after removing low order bending modes. The successful
completion of the DCT mirror demonstrates the engineering and performance of the support system, ability to finish
large aspheric surfaces using computer controlled polishing, and accuracy verification of surface measurements. In
addition to the DCT mirror, a 2-meter class flat was produced to an unprecedented accuracy of <10 nm-rms,
demonstrating the combined 1-m Fizeau interferometer and scanning pentaprism measurement techniques.
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We evaluated depth of subsurface damage on a ground surface of the ultra low expansion glass-ceramics
CLEARCERAMR®-Z HS (CC-Z HS) by Ohara Inc., which is one of the candidates for material for segmented
mirrors of the Thirty Meter Telescope. We made polishing spots of Magnetorheological Finishing on the
ground surface of CC-Z HS and measured exposed subsurface damage features on the spot surface. We also
studied on hydrofluoric acid etching of the CC-Z HS ground surface, which is expected to be an effective
method to remove a subsurface damage layer compared with time-consuming polishing. We etched small
ground surfaces of CC-Z HS and evaluated its uniformity.
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More and more astronomical telescopes use carbon fiber reinforced composites (CFRP). CFRP has high stiffness, high
strength, and low thermal expansion. However, they are not isotropic in performance. Their properties are direction
dependent. This paper discusses, in detail, the structural and thermal properties of carbon fiber structure members, such
as tubes, plates, and honeycomb sandwich structures. Comparisons are provided both from the structural point of view
and from the thermal point of view.
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To reduce the cost and increase the feasibility of the astronomical optical telescope, modern large optical telescope is
normally required to be as light as possible. Therefore lightweight mirror is always pursued by large telescopes
development. In this paper, a new type lightweight optical mirror blank, the evaluation of its technical feasibility and
the reduction of cost are introduced. For the purpose of applying active optics with this lightweight mirror blank, the
structural analysis, thermal analysis and optical performance simulation by the finite element method have been
presented.
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Successful launch and imagery from the Herschel Space Telescope has demonstrated a nominally in focus telescope.
There still remains a discrepancy between the prediction and measurement of the telescope back focal length prior to
launch. New material strain data has been applied to the structural/optical model of the telescope. The new data
significantly closed the gap between the previous optical test measurement and prediction. However, a discrepancy still
exists. Model results and techniques will be presented and discussed.
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This paper summarizes a comparison of pre- and post-flight optical performance on optical components (mostly
mirrors) from the Corrective Optics Space Telescope Axial Replacement (COSTAR) instrument and the Wide
Field Planetary Camera 2 (WF/PC 2) pickoff mirror. These measurements were carried out after both the
COSTAR and WF/PC 2 were retrieved from the Hubble Space Telescope in May of 2009 and returned to GSFC
in July of 2009. Both of these instruments had a highly UV-reflecting coating of Al with a MgF2 layer on
top for protection on their reflecting optics. We studied these in order to document the aging process on these
coatings while in space for more than 15.5 years. When compared to data before flight and witness coupons
kept on the ground, we find a severely degraded UV performance for the coatings that flew in space, particularly
at the Lyman-α wavelength. Based on similar observations seen earlier on the WF/PC1 POM, the current
degradation, of the latest optical components removed from HST, are a result of outgassing of substances such
as hydrocarbons and silicone from nearby hardware on the spacecraft and UV light that photo-polymerize those
materials on the mirror surfaces.
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Future space missions, such as SPICA and DARWIN, require large, light-weighted, and stiff mirrors with excellent
stability. During the past years, ECM fabricated a number of light-weighted mirrors. However, future large missions
require further advances with light-weighting, pushing this technology to the limit possible with recently developed
composite materials, new fabrication processes, and innovative designs.
In 2009, ECM took steps in this direction by fabricating successfully an ultra-lightweighted HB-Cesic® mirror
demonstrator 1 m in size. We demonstrated that HB-Cesic®, due to its extreme light-weighting capability and
manufacturing versatility, combined with high stiffness, high mechanical strength, and low CTE at cryogenic
temperatures, is able to fully meet the requirements of future large cryogenic space mirrors.
In this paper we describe the fabrication process of this demonstrator and its physical characteristics.
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During the past two years, ECM, Germany, together with Mitsubishi Electric Corporation (MELCO), Japan, developed a
new carbon-fiber-reinforced SiC material, called HB-Cesic®, which possesses superior mechanical and thermal
cryogenic properties compared to traditional Cesic®. This combination makes HB-Cesic® an excellent choice for large
cryogenic mirrors, which will be required for future scientific space missions, such as SPICA and DARWIN.
ESA contracted Thales Alenia Space (TAS), France, to design a super-lightweighted HB-Cesic® mirror with a diameter
of 600 mm, isostatic fixations, and a special astigmatism compensation device (ACD) for mirror shape control. The
mirror was manufactured by ECM, polished and coated by Société Européenne de Systèmes Optiques (SESO), France,
and tested to cryogenic temperatures by TAS. The measured wave-front error at ambient and cryogenic temperatures
demonstrated the excellent homogeneity of HB-Cesic® and TAS' expertise in mirror mounting. Furthermore, when
thermally actuated, the ACD exhibited perfect control of the mirror shape.
This success confirmed HB-Cesic®'s superior material properties and its applicability to future cryogenic space mirrors.
In this paper we describe the design and fabrication process of this cryogenic mirror and give test results at ambient and
cryogenic temperatures.
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Ultra-lightweight and high-accuracy CFRP (carbon fiber reinforced plastics) mirrors for space telescopes were fabricated
to demonstrate their feasibility for light wavelength applications. The CTE (coefficient of thermal expansion) of the all-
CFRP sandwich panels was tailored to be smaller than 1×10-7/K. The surface accuracy of mirrors of 150 mm in diameter
was 1.8 um RMS as fabricated and the surface smoothness was improved to 20 nm RMS by using a replica technique.
Moisture expansion was considered the largest in un-predictable surface preciseness errors. The moisture expansion
affected not only homologous shape change but also out-of-plane distortion especially in unsymmetrical compositions.
Dimensional stability due to the moisture expansion was compared with a structural mathematical model.
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CFRP (Carbon Fiber Reinforced Plastics) is the ideal material for space based mirror due to its low thermal expansion,
and high specific modulus. To expand the use of CFRP, we investigated the long-term stability of CFRP under humid
environment. CFRP mirror was made as precise as possible by using special class of material and adopting particular
design techniques. Dimensional stability of CFRP mirror was evaluated by nano-scale measurement. The factors which
cause out-of-plane deformation of the mirror is discussed.
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The European Solar Telescope (EST) is a European collaborative project to build a 4m class solar telescope in the
Canary Islands, which is now in its design study phase. The telescope will provide diffraction limited performance for
several instruments observing simultaneously at the Coudé focus at different wavelengths. A multi-conjugated adaptive
optics system composed of a tip-tilt mirror and several deformable mirrors will be integrated in the telescope optical
path.
The secondary mirror system is composed of the mirror itself (Ø800mm), the alignment drives and the cooling system
needed to remove the solar heat load from the mirror. During the design study the feasibility to provide fast tip-tilt
capabilities at the secondary mirror to work as the adaptive optics tip-tilt mirror is also being evaluated.
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During the experimental process of the spatial point's position detection, we analyze the advantages
and disadvantages of the classical method, and propose an improved method. First, we interpolate the
point's data to increase the size of the image. Then use the Zernike moment to detect the edge of the
point. Finally, we obtain a coordinate of the center of the point by using the ellipse fitting algorithm,
and take this coordinate as the position of the spatial point. Experimental results in laboratory show
that, the proposed detection method on sub-pixel level is realized to detect the spatial point's position
with high-accuracy. And experimental results of relative measurement with 100 meters outdoor show
that, the repeatable accuracy of this method can reach 0.12 pixel during the day and 0.05 pixel at night.
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A new method of attitude measurement of the calibration target based on machine vision is proposed in this paper.
Firstly, the internal parameters and external parameters of the CCD camera are calibrated by using the feature points on
the calibration target. Secondly, shoot the calibration target in different positions and gather any different image
information. Finally, measure the calibration target's attitude through reconstructing the positions of those feature points
which on the calibration target. Experiment results show that the standard deviation of the attitude measurement errors is
less than 10 arc-second. This method is an effective high-precision space attitude measurement algorithm which can
meet the engineering requirements.
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The method of 3D reconstruction from multi-view of the object with single camera is proposed in this paper. The images
of the planar pattern from different views are captured by the single camera. The internal and external parameters of the
camera are calculated precisely with Two-stage camera calibration method. On this basis, the image points of calibration
pattern can be matched and reconstructed. Experimental results show that: the flatness of reconstructed planar pattern is
about 0.006mm; the position error of sign is about 0.012mm. The proposed method is high-accuracy, simple, flexible and
effective.
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We discuss the use of a Faro Arm - a portable coordinate measuring machine - in the re-alignment and testing of the
Southern African Large Telescope's Spherical Aberration Corrector. In pushing this versatile tool to its limits, we have
arrived at a better understanding of its true capabilities and developed ways to compensate for some of its weaknesses. It
is possible to achieve excellent results (~5 microns) when making relative measurements and keeping the Arm's
orientation relatively constant. The Faro is also extremely useful in providing live feedback while making fine
adjustments. However, single measurements of the same position are considerably less reliable (~50 microns) when the
Arm is operated in widely different orientations. If the latter is unavoidable, one may largely counteract this by taking
continuous streams of measurements (1000 readings) for a given point while exercising the Arm as much as possible in
order to average the encoder errors. Various other techniques and accessories that we have found useful, as well as some
painful lessons, are described here and a few examples are given to demonstrate how we have used a Faro Arm in our
challenging optical alignment project.
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Typical radio telescopes have the primary reflector surface which is composed of several single panels that have
dimensions of a meter a side. The manufacturing of these radio panels yield a micrometric precision over the volume on
the single panel, hence the surface roughness of the panels can be measured with very high accuracy by means of the low
coherence interferometry (LCI) technique which reaches micrometric spatial and depth resolution and has the advantage
of being contact-less.
We have developed a multi-channel partially coherent light interferometer to realize non contact 3D surface topography.
The technique is based on the LCI principle, for which a bi-dimensional sensor - a CMOS - has been developed to
directly acquire images. Tri-dimensional measures are recovered with a single scanning along the depth direction in a
millimetric range, and every single pixel of the bi-dimensional sensor measures a point on the object, this allows a fast
analysis in real time on square centimeter areas.
In this paper we show the results obtained by applying the LCI technique method to analyze the surface roughness of the
panels of a large radio antenna of 64 m of width and used for astronomical observations at 100 GHz; by measuring their
3D structure at micrometric resolution it is possible to verify their fabrication errors.
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The Large Synoptic Survey Telescope (LSST) utilizes a three-mirror design in which the primary (M1) and tertiary (M3)
mirrors are two concentric aspheric surfaces on one monolithic substrate. The substrate material is Ohara E6 borosilicate
glass, in a honeycomb sandwich configuration, currently in production at The University of Arizona's Steward
Observatory Mirror Lab. In addition to the normal requirements for smooth surfaces of the appropriate prescriptions, the
alignment of the two surfaces must be accurately measured and controlled in the production lab. Both the pointing and
centration of the two optical axes are important parameters, in addition to the axial spacing of the two vertices. This
paper describes the basic metrology systems for each surface, with particular attention to the alignment of the two
surfaces. These surfaces are aspheric enough to require null correctors for each wavefront. Both M1 and M3 are concave
surfaces with both non-zero conic constants and higher-order terms (6th order for M1 and both 6th and 8th orders for M3).
M1 is hyperboloidal and can utilize a standard Offner null corrector. M3 is an oblate ellipsoid, so has positive spherical
aberration. We have chosen to place a phase-etched computer-generated hologram (CGH) between the mirror surface
and the center-of-curvature (CoC), whereas the M1 null lens is beyond the CoC. One relatively new metrology tool is the
laser tracker, which is relied upon to measure the alignment and spacings. A separate laser tracker system will be used to
measure both surfaces during loose abrasive grinding and initial polishing.
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The Advanced Wavefront Sensing and Control Testbed (AWCT) is built as a versatile facility for developing and
demonstrating, in hardware, the future technologies of wavefront sensing and control algorithms for active optical
systems. The testbed includes a source projector for a broadband point-source and a suite of extended scene targets, a
dispersed fringe sensor, a Shack-Hartmann camera, and an imaging camera capable of phase retrieval wavefront
sensing. The testbed also provides two easily accessible conjugated pupil planes which can accommodate active optical
devices such as fast steering mirror, deformable mirror, and segmented mirrors. In this paper, we describe the testbed
optical design, testbed configurations and capabilities, as well as the initial results from the testbed hardware
integrations and tests.
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Phase retrieval is an image-based wavefront sensing process, used to recover phase information from defocused
stellar images. Phase retrieval has proven to be useful for diagnosis of optical aberrations in space telescopes,
calibration of adaptive optics systems, and is intended for use in aligning and phasing the James Webb Space
Telescope. This paper describes a robust and accurate phase retrieval algorithm for wavefront sensing, which has
been successfully demonstrated on a variety of testbeds and telescopes. Key features, such as image preprocessing,
diversity adaptation, and prior phase nulling, are described and compared to other methods. Results demonstrate
high accuracy and high dynamic range wavefront sensing.
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Optical cophasing has a key role in ensuring that segmented mirror telescopes reach their best performance. To
measure and correct segments misalignment it is necessary to have a wavefront sensor (WFS) in the telescope
optical path. All the cophasing WFS suffer the phase ambiguity problem that limits the piston error measurements to a unit of wavelength. To overcome this problem we have developed a new cophasing technique based
on the wavelength sweep.
This paper will present the results of laboratory and on-sky tests of this technique, comparing them with
the expected performance obtained in a previous work through numerical simulations. The laboratory test was
carried out on the Active Phasing Experiment bench at ESO premises in Garching. We measured wavefront
piston errors up to 15μm with an accuracy better than 0.25μm on a pupil conjugate segmented mirror using
the Pyramid Phasing Sensor (PYPS) and a commercial tunable filter. We tested the possibility of propagating
the differential piston measurements over the segmented mirror to cophase it, obtaining a residual surface error
less than 0.2μm rms. The first on-sky test of the WST was carried out at William Hershel Telescope (WHT)
using the NAOMI segmented mirror. We checked the effects of atmospheric turbulence on the measurements of
large piston errors up to 15um wavefront and it was obtained an accuracy of 0.5μm, which is in agreement with
simulation.
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Poster Session: Telescope Structure and Mechanical Design
The key factor, which influences the size of future radio aperture synthesis telescope projects, is the cost of the antenna
dish. Lower antenna dish cost will allow more antennas to be built, providing more baselines and improving the
telescope sensitivity and image quality. In this paper, various methods of lowering the dish structure cost, including
using pre-stressed beam members, are discussed and, finally, an innovative low cost small antenna dish is proposed
which relies mostly on commercial off-the-shelf components.
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The 2.6-m primary mirror of the VST telescope is equipped with an active optics system in order to correct low-order
aberrations, constantly monitoring the optical quality of the image and controlling the relative position and the shape of
the optical elements. Periodically an image analyser calculates the deviation of the image from the best quality. VST is
equipped with both a Shack-Hartmann in the probe system and a curvature sensor embedded in the OmegaCAM
instrument. The telescope control software decomposes the deviation into single optical contributions and calculates the
force correction that each active element has to perform to achieve the optimal quality. The set of correction forces, one
for each axial actuator, is computed by the telescope central computer and transmitted to the local control unit of the
primary mirror system for execution. The most important element of the VST active optics is the primary mirror, with its
active support system located within the primary mirror cell structure. The primary mirror support system is composed
by an axial and a lateral independent systems and includes an earthquake safety system. The system is described and the
results of the qualification test campaign are discussed.
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The VST telescope is equipped with an active optics system based on a wavefront sensor, a set of axial actuators to
change the primary mirror shape and a secondary mirror positioner stage. The secondary mirror positioning capability
allows the correction of defocus and coma, caused by incorrect relative positions of the two mirrors arising from the
deformation of the telescope tube and of the optical train under the effect of gravity and thermal espansion. Periodically
the image analyser calculates the deviation of the image from the best quality and the telescope control software
decomposes the deviation into the single optical contributions. The new position and orientation of the secondary mirror
is computed by the telescope control software and transmitted to the secondary mirror support system for execution. The
secondary mirror positioner is a hexapod, i.e. a parallel robot with a mobile platform moved by six linear actuators acting
simultaneously. This paper describes the secondary mirror support system and the qualification test campaign performed
both in laboratory and at the telescope.
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The VST primary mirror is a 2.6-m meniscus made of Astro-Sitall. An active optics system is implemented to correct
surface errors due to manufacturing or induced by gravity and temperature changes. The primary mirror is axially
supported by 84 supports disposed in four concentric rings. Three of the supports, symmetrically placed and much stiffer
than the other ones, define the axial plane of the primary mirror acting as fixed points. The remaining 81 supports are
force controlled actuators, used to change the shape of the mirror according to wavefront measurements in closed loop
operation, or to a look-up table in open loop. This paper describes the solutions adopted for the axial actuator, as well as
the test campaign to assess their performance and degree of reliability.
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We studied the thermal effects on the 32 m diameter radio-telescope managed by the Institute of Radio Astronomy
(IRA), Medicina, Bologna, Italy. The preliminary results show that thermal gradients deteriorate the pointing
performance of the antenna.
Data has been collected by using: a) two inclinometers mounted near the elevation bearing and on the central part of the
alidade structure; b) a non contact laser alignment optical system capable of measuring the secondary mirror position; c)
twenty thermal sensors mounted on the alidade trusses.
Two series of measurements were made, the first series was performed by placing the antenna in stow position, the
second series was performed while tracking a circumpolar astronomical source.
When the antenna was in stow position we observed a strong correlation between the inclinometer measurements and the
differential temperature. The latter was measured with the sensors located on the South and North sides of the alidade,
thus indicating that the inclinometers track well the thermal deformation of the alidade.
When the antenna pointed at the source we measured: pointing errors, the inclination of the alidade, the temperature of
the alidade components and the subreflector position. The pointing errors measured on-source were 15-20 arcsec greater
than those measured with the inclinometer.
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The very short slew times and resulting high inertial loads imposed upon the Large Synoptic Survey Telescope (LSST) create new challenges to the primary mirror support actuators. Traditionally large borosilicate mirrors are supported by pneumatic systems, which is also the case for the LSST. These force based actuators bear the weight of the mirror and provide active figure correction, but do not define the mirror position. A set of six locating actuators (hardpoints) arranged in a hexapod fashion serve to locate the mirror. The stringent dynamic requirements demand that the force actuators must be able to counteract in real time for dynamic forces on the hardpoints during slewing to prevent excessive hardpoint loads. The support actuators must also maintain the prescribed forces accurately during tracking to maintain acceptable mirror figure. To meet these requirements, candidate pneumatic cylinders incorporating force feedback control and high speed servo valves are being tested using custom instrumentation with automatic data recording. Comparative charts are produced showing details of friction, hysteresis cycles, operating bandwidth, and temperature dependency. Extremely low power actuator controllers are being developed to avoid heat dissipation in critical portions of the mirror and also to allow for increased control capabilities at the actuator level, thus improving safety, performance, and the flexibility of the support system.
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Magdalena Ridge Observatory Interferometer (MROI) comprises an array of up to ten (10) 1.4m diameter mirror
telescopes. Each of these ten telescopes will be housed inside a Unit Telescope Enclosure (UTE) which can be relocated,
with the telescope inside, to any of 28 stations arranged in a "Y" configuration. These stations comprise fixed
foundations with utility and data connections. There are four standard array configurations, the most compact of which
one has less than 350 mm of space between the enclosures.
This paper describes the relocation systems that were evaluated, including a rail based system, wheels or trolley fixed to
the bottom of the enclosure, and various lifting mechanisms, all of which were analyzed to determine their performances
related to the requirements. Eventually a relocation system utilizing a modified reachstacker (a transporter used to handle
freight containers) has been selected. The reachstacker is capable of manoeuvring between and around the enclosures, is
capable of lifting the combined weight of the enclosure with the telescope (40tons), and can manoeuvre the enclosure
with minimal vibrations. A rigorous testing procedure has been performed to determine the vibrations induced in a
dummy load in order to guarantee the safety of optics that must remain on the nasmyth table during the relocation.
Finally we describe the lifting system, constituted by hydraulic jacks and locating pins, designed to lift and lower the
enclosure and telescope during the precise positioning of the telescopes in the various stations.
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As direct drive technology is finding their way into telescope drive designs for its many advantages, it would push to
more reliable and cheaper solutions for future telescope complex motion system. However, the telescope drive system
based on the direct drive technology is one high integrated electromechanical system, which one complex
electromechanical design method is adopted to improve the efficiency, reliability and quality of the system during the
design and manufacture circle. The telescope is one ultra-exact, ultra-speed, high precision and huge inertial instrument,
which the direct torque motor adopted by the telescope drive system is different from traditional motor. This paper
explores the design process and some simulation results are discussed.
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In the frame of the E-ELT-EAGLE instrument phase A studies, we designed a convex VCM able to compensate
for the focus variation on the Laser Guide Star (LGS) wavefront sensor, due to the elevation of the telescope
and the fixed sodium layer altitude. We present an original optical design including this active convex mirror,
providing a large sag variation on a spherical surface with a 120mm clear aperture, with an optical quality
better than lambda/5 RMS up to 820μm of sag and better than lambda/4 RMS up to 1000μm of sag. Finite
element analysis (FEA) allowed an optimisation of the mirror's variable thickness distribution to compensate
for geometrical and material non linearity. Preliminary study of the pre-stressing has also been performed by
FEA, showing that a permanent deformation remains after removal of the loads. Results and comparison with
the FEA are presented in the article of F.Madec et al (AS10-7736-119, this conference), with an emphasis on
the system approach.
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The need for both high quality images and light structures is a constant concern in the conception of space telescopes.
The goal here is to determine how an active optics system could be embarked on a satellite in order to correct the wave
front deformations of the optical train. The optical aberrations appearing in a space environment are due to mirrors'
deformations, with three main origins: the thermal variations, the weightlessness in space with respect to the
Assemblage, Integration and Testing (AIT) conditions on ground and the use of large weightlighted primary mirrors.
We are developing a model of deformable mirror as minimalist as possible, especially in term of number of actuators,
which is able to correct the first Zernike polynomials in the specified range of amplitude and precision. Flight constraints
as weight, volume and power consumption have to be considered. Firstly, such a system is designed according to the
equations from the elasticity theory: we determine the geometrical and mechanical characteristics of the mirror, the
location of the forces to be applied and the way to apply them. The concept is validated with a Finite Element Analysis
(FEA), allowing optimizing the system by taking into account parameters absent from the theory. At the end of the
program the mirror will be realized and characterized in a representative optical configuration.
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The whole LAMOST(Large-sky-area multi-object fiber spectroscopic telescope) project achieved successfully last year
and has observed properly. As the high precision and large scale modern measuring instrument, laser tracker is widely
used in various field environment. But in the process of LAMOST building, the measurement requires high precision in
most cases especially for focal plane system measuring. This paper mainly focuses on the focal plane measurement, to
testify the feasibility for using laser tracker to measure the focal plane plate. And a lot of experiment results and
analysis will be mentioned in the paper. There are three laser tracker producers in the world and the main models with
the three producers have all compared in detail. Furthermore, the special calibration scheme for the laser tracker will be
discussed, and tries to improve the precision and stability of the laser tracker measuring system in the LAMOST field
environment.
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To enable the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), the McDonald Observatory (MDO) and
the Center for Electro-mechanics (CEM) at the University of Texas at Austin are developing a new HET tracker in
support of the Wide-Field Upgrade (WFU) and the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). The
precision tracker is required to maintain the position of a 3,100 kg payload within ten microns of its desired position
relative to the telescope's primary mirror. The hardware system to accomplish this has ten precision controlled
actuators. Prior to installation on the telescope, full performance verification is required of the completed tracker in
CEM's lab, without a primary mirror or the telescope's final instrument package. This requires the development of a
laboratory test stand capable of supporting the completed tracker over its full range of motion, as well as means of
measurement and methodology that can verify the accuracy of the tracker motion over full travel (4m diameter circle,
400 mm deep, with 9 degrees of tip and tilt) at a cost and schedule in keeping with the HET WFU requirements. Several
techniques have been evaluated to complete this series of tests including: photogrammetry, laser tracker, autocollimator,
and a distance measuring interferometer, with the laser tracker ultimately being identified as the most viable method.
The design of the proposed system and its implementation in the lab is presented along with the test processes, predicted
accuracy, and the basis for using the chosen method*.
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The SOAR telescope fast tip-tilt tertiary mirror, was delivered by the Goodrich Optical and Space Systems Division,
Danbury, CT, and integrated into the SOAR optical system in 2004. It consist of a plane, light weighted 655×470 mm
elliptical mirror, controllable over a range of ±1 mrad, in two axes, with a required position loop bandwidth of 50 Hz. It
operates using the signal from a fast read-out guide camera to generate position commands, in an outer loop fashion.
The original tertiary mirror controller consisted of several analog circuit boards, incorporating the position control loop
compensation, and power amplifiers. This system was limited by the difficulty of making any modifications, to optimize
the control loop, and meet the required bandwidth. The analog controller was replaced with a digital controller based on
a National Instruments Compact RIO/FPGA device. This allows the full optimization of the control system, and also
allows closing the torque (acceleration) loop using the optical feedback of the guide signal alone, which should result in
even higher performance. This paper will describe the models, design, and performance tests, of the new digital control
system.
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In comparison to mechanical cryo-coolers, liquid nitrogen cooling has the double advantage to be free of vibration and to
remain not affected by power failure.
The paper reports about a very compact cryostat using a continuous circulation of liquid nitrogen which is provided from
an external storage tank. Since years, this cryostat is intensively used on the ESO VLT to cool either optical or Infra Red
detectors.
After an introduction presenting the principle, the paper reports the performance of the cryostat recorded over many
years of utilization. We also present a few additional developments which allow the use of the cryostat for more exotic
applications such that Nasmyth rotating instruments or extremely stable radial velocity spectrograph. With the
construction of MUSE, a new era has started for this cryostat. The large multi IFU instrument requires 24 cryostats. The
last chapter of the paper describes this futurist system which is close to completion.
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Since the last decade, most of the large infrared instruments are kept at operating cryogenic temperature using
mechanical cryo-coolers. Generally Gifford MacMahon Closed Cycle Coolers or Pulsed Tubes are doing this duty.
These coolers are well dimensioned to keep the instrument and the detector at a sufficiently low operating temperature.
Using the only cooling power provided by the steady state mechanical cryo-coolers would lead to several days for the
initial cooling down. Therefore an additional cooling has to be used to allow a reasonable cooling time.
The present paper describes the liquid nitrogen continuous flow cooling system developed at ESO for ISAAC. During
the past years, this system has also been used successfully for a number of VLT instruments (CRIRES, HAWK-I..).
After a short comparison with the more common technique using an instrument internal tank, we list in detail the various
developments which have been required to get the continuous flow working in a reliable and efficient way.
This paper also presents the advantages making this technology as a potential very attractive way to replace definitively
mechanical coolers in most of the cases.
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HAWK-I is a near-infrared imager with a relatively large field of view. Two filter wheels with 6 positions each offer a
choice of 10 filters. The filters are directly in front of the detector, a mosaic of 2 × 2HAWAII 2RG 2048×2048 pixels
detectors. A rather high positioning reproducibility (< 6 arc sec) is required in order to avoid any disagreement caused by
subtraction of eventual fix pattern on the filters.
The document describes various drive systems which have been tested in order to reach the specified positioning
reproducibility. This includes an interesting dissipation free locking system combining electro magnet and permanent
magnet. Every solution is discussed and the performances measured in the laboratory during a long campaign of test are
exposed. We also address the choice of other critical components like the ball bearings, mounting of the filters and
cooling of the wheels.
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OmegaCAM is a wide field camera housing a mosaic of 32 CCD detectors. For the optimal trade-off between dark
current, sensitivity, and cosmetics, these detectors need to be operated at a temperature of about 155 K. The detectors
mosaic with a total area of 630 cm2 directly facing the Dewar entrance window, is exposed to a considerable radiation
heat load. This can only be achieved with a very performing cooling system.
The paper describes the cooling system, which is build such that it makes the most efficient use of the cooling power of
the liquid nitrogen. This is obtained by forcing the nitrogen through a series of well designed and strategically distributed
heat exchangers.
Results and performance of the system recorded during the laboratory system testing are reported as well. In addition to
the cryogenic performance, the document reports also about the overall performance of the instrument including long
term vacuum behavior.
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This paper describes the development of high-cooling power systems by making use of multiple cold head operation
with a minimized number of compressor units. These advanced cooling systems were investigated for optimization and
their Carnot efficiencies were analyzed. Test series were performed to monitor and rank some of their critical operation
parameters. Operating envelopes for different cold head / compressor configurations were defined for applications in
various VLT instruments. This new concept of providing high pressure helium as a service point for a large number of
detached cold heads is a first step towards a new cryogenic facility concept for the E-ELT.
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The start of the new generation of giant telescopes opens a good opportunity to re-assess the cryogenic cooling of the
instruments and detectors. An analysis has been carried out comparing three different technologies: Mechanical cryocoolers,
helium forced flow and open liquid nitrogen cooling. The most different aspects from the running cost to the
reliability and technology readiness have been compared in order to establish a fair ranking. The first part of the paper
will present in detail the result of this analysis.
Based on this study and the various experiences collected over more than 25 years and a large number of cryogenic
instruments, a strategy is elaborated for the cryogenic cooling of the E-ELT (European Extremely Large Telescope)
instrument suite.
The challenge consists in providing various cryogenic temperatures (from 10 K to 240 K) at various locations. This
should be done in the most efficient way with the minimum of disturbances (low vibration, low thermal dissipation...). A
discussion presents the advantages of the selected solution.
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We present a first design study of the shutter mechanism to be implemented on the visible channel of the
Euclid imager. The main functionality of the shutter is to obscure the light during the detector read-out and
flat field calibration. Hence, the major design drivers are the number of open/close cycles of 160,000 and the
opening/closing time of 5 sec without introducing a too large uncompensated momentum disturbance. The
current design foresees to use two fully redundant actuators, which drive the shutter via a lever system. In case
of an actuator failure, the blocked actuator can be disengaged via a fail-safe system.
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The Dark Energy Survey is a Stage III Dark Energy Experiment that will obtain cosmological parameters by combining
four observational techniques; Galaxy Clusters, Weak Lensing, Type Ia Supernovae and Baryon Acoustic Oscillations.
The observations will be performed with a new wide field camera (DECam) that will be placed on the Blanco 4 m
telescope at CTIO. Here we describe the large format (600 mm clear aperture) Filter Changer Mechanism (FCM) for the
Dark Energy Survey Camera (DECam). The FCM, based on the Pan-STARRS design, is the largest ever constructed.
Fabrication of the filter changer has been completed and it has been tested under realistic conditions.
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The Dark Energy Camera (DECam) is the new wide field prime-focus imager for the Blanco 4m telescope at CTIO.
This instrument is a 2.2 sq. deg. camera with a 45 cm diameter focal plane consisting of 62 2k × 4k CCDs and 12 2k × 2k
CCDs and was developed for the Dark Energy Survey that will start operations at CTIO in 2011. DECam includes the
vessel shell, the optical window cell, the CCDs with their readout electronics and vacuum interface, the focal plane
support plate and its mounts, and the cooling system and thermal controls. Assembly of the imager, alignment of the
focal plane and installation of the CCDs are described. During DECam development a full scale prototype was used for
multi-CCD readout tests. This test vessel went through several stages as the CCDs and related hardware progressed
from early prototypes to final production designs.
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The Dark Energy Camera (DECam) is the new wide field prime-focus imager for the Blanco 4m telescope at CTIO. This
instrument is a 3 sq. deg. camera with a 45 cm diameter focal plane consisting of 62 2k × 4k CCDs and 12 2k × 2k CCDs
and was developed for the Dark Energy Survey that will start operations at CTIO in 2011. The DECam CCD array is
inside the imager vessel. The focal plate is cooled using a closed loop liquid nitrogen system. As part of the development
of the mechanical and cooling design, a full scale prototype imager vessel has been constructed and is now being used
for Multi-CCD readout tests. The cryogenic cooling system and thermal controls are described along with cooling
results from the prototype camera. The cooling system layout on the Blanco telescope in Chile is described.
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Details of a novel lens mount are described. The design makes use of existing concepts in design and manufacturing to produce an elegant method for establishing and maintaining accurate lens placement over a broad range of temperature. Here lenses are centered by multiple roll-pin shaped flexures precisely machined into the mount. Like other flexure mounts, the roll-pin flexures provide radial compliance to accommodate the difference in radial expansion between the lens and mount. However, the cylindrical flexure geometry is easily multiplexed and allows reference features for axial placement, centration in a barrel, and mount-to-mount stacking to be more readily integrated in a single monolithic lens cell. This eases manufacture and improves accuracy. In this paper, the concept, analysis, and design details for the roll-pin flexure mount are presented, along with examples of their use with a variety of lens materials, diameters (25 mm to 375 mm), and temperatures ranges (ambient to cryogenic).
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Operating on 6 interferometric baselines, i.e. using all 4 UTs, the 2nd generation VLTI instrument GRAVITY will
deliver narrow angle astrometry with 10μas accuracy at the infrared K-band.
Within the international GRAVITY consortium, the Cologne institute is responsible for the development and
construction of the two spectrometers: one for the science object, and one for the fringe tracking object.
Optically two individual components, both spectrometers are two separate units with their own housing and interfaces
inside the vacuum vessel of GRAVITY. The general design of the spectrometers, however, is similar. The optical layout
is separated into beam collimator (with integrated optics and metrology laser injection) and camera system (with
detector, dispersive element, & Wollaston filter wheel). Mechanically, this transfers to two regions which are separated
by a solid baffle wall incorporating the blocking filter for the metrology Laser wavelength. The optical subunits are
mounted in individual rigid tubes which pay respect to the individual shape, size and thermal expansion of the lenses.
For a minimized thermal background, the spectrometers are actively cooled down to an operating temperature of 80K in
the ambient temperature environment of the GRAVITY vacuum dewar. The integrated optics beam combiner and the
metrology laser injection, which are operated at 200/240K, are mounted thermally isolated to the cold housing of the
spectrometers.
The optical design has shown that the alignment of the detector is crucial to the performance of the spectrometers.
Therefore, in addition to four wheel mechanisms, six cryogenic positioning mechanisms are included in the mechanical
design of the detector mount.