A detailed Computational Fluid Dynamics (CFD) model for the Giant Magellan Telescope (GMT) telescope has been developed and used to simulated and analyze the aero-optical environment around the observatory. The developed model accounts for the major observatory components such as the primary (M1) and secondary (M2) mirrors, the M2 supporting truss, other subcomponents of the telescope mount, and enclosure building along with the auxiliary and site support buildings on the summit. A large topographical area around the installation site is included. This study evaluates three different lower enclosure designs; a closed soffit, an open soffit and a perforated ring-wall (partially closed soffit). Timevarying CFD simulations provide detailed flow and temperature fields along the optical path, which are subsequently used to compute optical parameters such as Optical Path Difference (OPD) maps and Point Source Sensitivity normalized (PSSn), the GMT Image Quality (IQ) metric. Results show that enclosure-induced turbulent flow patterns and refractive index variations have a greater influence on optical performance compared to flow and thermal behavior external to the enclosure. Instantaneous and mean PSSn values obtained for the three soffit configurations show minor differences, indicating that the lower enclosure design has minimal impact on observatory optical performance for the simulated operating conditions.
The Giant Magellan Telescope (GMT) is currently planned for construction at Las Campanas Peak in northern Chile. Part of the next generation of extremely large telescopes, GMT will be one of the most powerful ground-based telescopes in operation in the world. Due to the larger aperture envisioned for GMT, characterization and control of the air flow entering and circulating within the enclosure will be required to maintain the highest possible image quality. Aero-thermal interactions between the site topography, enclosure, internal systems, and optics are complex. A key parameter for image quality is the thermal gradient between the terrain and the air mass entering the enclosure, and how quickly that gradient can be dissipated to equilibrium. Because the thermal gradients are highest near the ground, an important function of the GMT enclosure is to minimize the flow of ground-layer air entering the enclosure. By doing so, a more uniform air density above the telescope will enable higher image quality. <p> </p>The design of the GMT lower enclosure is driven by equipment storage and access requirements but also directly impacts the origin and quality of the air entering the enclosure aperture. To ensure the highest quality GMT optical performance, Computational Fluid Dynamics (CFD) models and specialized analyses are utilized to evaluate several lower enclosure designs for their ability to limit the amount of ground-layer air entering the enclosure aperture. Lower enclosure designs with traditional solid outer walls promote the formation of “necklace” vortices, which tend to direct near-surface air, containing steep thermal gradients, into the enclosure aperture, potentially reducing image quality. Modifications to the lower enclosure, such as perforating the outer walls, are shown to suppress these necklace vortices at the expense of added structural complexity and/or reduced internal storage space. Initial isothermal CFD simulations defined the minimum height above terrain reached by the flow-path upwind of the observatory as a proxy to characterize the quality of air entering the enclosure, with lower heights associated with steeper thermal gradients. Based on these results, the most promising designs are further refined and subjected to additional higher fidelity CFD analyses, which includes a terrestrial thermal boundary layer. These simulations are also surveyed to quantify the aero-thermal environment along telescope optical paths, permitting evaluation and comparison of the predicted optical performance of the final candidate enclosure designs. Results from preliminary water tunnel testing of select lower-enclosure designs have increased our confidence in the CFD simulations.
The Giant Magellan Telescope (GMT), one of three next-generation extremely large telescopes (ELTs), will have a 25.4- meter diameter effective aperture, and will be located on the summit of Cerro Las Campanas in Chile. Developing a new observatory for cutting-edge science operations and a 50-year lifespan poses challenges that have resulted in competing design concepts. This paper discusses the concepts that have been adopted in the GMT site master plan, including designs for the site infrastructure, telescope enclosure, and facilities. The GMTO site has been in active construction since 2015, and in the past two years has completed important steps in site development including completion of residential and office facilities, road improvements, and other necessary infrastructure to support upcoming work. This paper concludes with an overview on managing design and construction simultaneously.
The Giant Magellan Telescope project is proceeding with design, fabrication, and site construction. The first of the seven required 8.4-m primary mirror segments is completed and in storage, three segments are in various stages of grinding and polishing, and the fifth segment has been cast. Industry contracts are underway to complete the design of the telescope structure. Residence buildings and other facilities needed to support construction at the Las Campanas site in Chile are complete. Hard rock excavation is imminent in preparation for the pouring of concrete for the telescope pier and other foundations. Computational fluid dynamics analysis is informing the design of the telescope enclosure, and further construction work packages are being readied for tender. Seismic design considerations have resulted in the incorporation of a seismic isolation system into the telescope pier, as well as modifications to the primary mirror support system. Designs for the fast-steering and adaptive secondary mirrors, science instruments, and other subsystems are maturing. Prototyping is underway in various aspects, including on-sky testing of wavefront sensing and control elements, and the telescope metrology system. Our fabrication and construction schedule calls for engineering first light with a subset of primary mirror segments in late 2023, with buildout to the full configuration occurring in stages, paced by the availability of primary mirror segments and other components.
Telescope enclosure azimuth rotation systems have traditionally been supported by custom bogies with steel wheels and steel rails, with mixed results in terms of long-term reliability and performance. Because the enclosure azimuth rotation mechanisms are vital for the operational success of all telescopes, and because the scale of the Giant Magellan Telescope (GMT) enclosure will exceed that of all enclosures now in existence, the GMT project team has explored alternative solutions for enclosure rotation in search of cost, reliability, and maintainability benefits. Four concepts are studied: railway bogies, ring crane bogies, segmented slewing bearings, and THK curved linear bearings. All four concepts are highly developed systems engineered to meet specific design objectives and performance requirements, some objectives of which overlap those of the GMT enclosure azimuth rotation system; however, in all four instances, significant customization or development of an altogether new product would be required for fulfilment of the GMT performance requirements.
The Giant Magellan Telescope (GMT) is an Extremely Large Telescope (ELT) class observatory set to make history as one of the largest telescopes ever built. Vast improvements in the fields of optics, control systems, and mirror fabrication technologies have facilitated correspondingly drastic increases in the size and presence of ground-based telescopes previously thought to be impossible. Size for these observatories has increased to the point where conventional approaches impart seismic demands on the telescope structure and optics that are unmanageable. With this, a refined approach involving base isolation is being designed to provide seismic protection of a sensitive, invaluable instrument that will revolutionize our understanding of the universe.
The Giant Magellan Telescope (GMT) is planned for construction at a summit of Cerro Las Campanas at the Los Campanas Observatory (LCO) in Chile. GMT will be the most powerful ground-based telescope in operation in the world. Aero-thermal interactions between the site topography, enclosure, internal systems, and optics are complex. A key parameter for optical quality is the thermal gradient between the terrain and the air entering the enclosure, and how quickly that gradient can be dissipated to equilibrium. To ensure the highest quality optical performance, careful design of the telescope enclosure building, location of the enclosure on the summit, and proper venting of the airflow within the enclosure is essential to minimize the impact of velocity and temperature gradients in the air entering the enclosure. <p> </p>High-fidelity Reynolds-Averaged Navier Stokes (RANS) Computational Fluid Dynamics (CFD) analysis of the GMT, enclosure, and LCO terrain is performed to study (a) the impact of either an open or closed enclosure base soffit external shape design, (b) the effect of telescope/enclosure location on the mountain summit, and (c) the effect of enclosure venting patterns. Details on the geometry modeling, grid discretization, and flow solution are first described. Then selected computational results are shown to quantify the quality of the airflow entering the GMT enclosure based on soffit, site location, and venting considerations. Based on the results, conclusions are provided on GMT soffit design, site location, and enclosure venting. The current work is not used to estimate image quality but will be addressed in future analyses as described in the conclusions.
In the era of extremely large telescopes (ELTs), with telescope apertures growing in size and tighter image quality requirements, maintaining a controlled observation environment is critical. Image quality is directly influenced by thermal gradients, the level of turbulence in the incoming air flow and the wind forces acting on the telescope. Thus any ELT enclosure must be able to modulate the speed and direction of the incoming air and limit the inflow of disturbed ground-layer air. However, gaining an a priori understanding of the wind environment’s impacts on a proposed telescope is complicated by the fact that telescopes are usually located in remote, mountainous areas, which often do not have high quality historic records of the wind conditions, and can be subjected to highly complex flow patterns that may not be well represented by the traditional analytic approaches used in typical building design. As part of the design process for the Giant Magellan Telescope at Cerro Las Campanas, Chile; the authors conducted a parametric design study using computational fluid dynamics which assessed how the telescope’s position on the mesa, its ventilation configuration and the design of the enclosure and windscreens could be optimized to minimize the infiltration of ground-layer air. These simulations yielded an understanding of how the enclosure and the natural wind flows at the site could best work together to provide a consistent, well controlled observation environment. Future work will seek to quantify the aerothermal environment in terms of image quality.
The Giant Magellan Telescope Project is in the construction phase. Production of the primary mirror segments is underway with four of the seven required 8.4m mirrors at various stages of completion and materials purchased for segments five and six. Development of the infrastructure at the GMT site at Las Campanas is nearing completion. Power, water, and data connections sufficient to support the construction of the telescope and enclosure are in place and roads to the summit have been widened and graded to support transportation of large and heavy loads. Construction pads for the support buildings have been graded and the construction residence is being installed. A small number of issues need to be resolved before the final design of the telescope structure and enclosure can proceed and the GMT team is collecting the required inputs to the decision making process. Prototyping activities targeted at the active and adaptive optics systems are allowing us to finalize designs before large scale production of components begins. Our technically driven schedule calls for the telescope to be assembled on site in 2022 and to be ready to receive a subset of the primary and secondary mirror optics late in the year. The end date for the project is coupled to the delivery of the final primary mirror segments and the adaptive secondary mirrors that support adaptive optics operations.
Performance of the GMT azimuth drive system is vital for the operation of the telescope and, as such, all components subject to wear at the drive interface merit a high level of scrutiny for achieving a proper balance between capital costs, maintenance costs, and the risk for downtime during planned and unplanned maintenance or replacement procedures. Of particular importance is the interface between the azimuth wheels and rail, as usage frequency is high, the full weight of the enclosure must be transferred through small patches of contact, and replacement of the rail would pose a greater logistical challenge than the replacement of smaller components such as bearings and gearmotors. This study investigates tradeoffs between various wheel-rail and roller-track interfaces, including performance, complexity, and anticipated wear considerations. First, a survey of railway and overhead crane industry literature is performed and general detailing recommendations are made to minimize wear and the risk of rolling contact fatigue. Second, Adams/VI-Rail is used to simulate lifetime wear of four specific configurations under consideration for the GMT azimuth wheel-rail interface; all studied configurations are shown to be viable, and their relative merits are discussed.
The Giant Magellan Telescope (GMT) will have a 25.4-meter diameter effective aperture, and is one the three currently planned next generation extremely large telescopes (ELTs). The GMT will be located at the summit of Cerro Campanas at the Las Campanas Observatory (LCO) in Chile, one the world’s best observing sites. This paper provides an overview of the site master plan comprising site infrastructure, enclosure, and facilities, and outlines the analysis of alternative trade studies that will lead to the final design. Also presented is an update of the site infrastructure development and preconstruction activities currently underway that will be completed prior to the beginning of enclosure construction near the end of 2016.
The suspension and rotation systems (typically called bogies) for Extremely Large Telescope (ELT) enclosures will carry structures that are 2-3 times greater in diameter and much heavier than enclosures for the previous generation of 6-10m telescopes. Via on-site visits and/or engineering documentation, we have surveyed eleven optical, infrared, and submillimeter 3-15m telescope enclosures, and report on key design features of the suspension and rotation systems, including wheel and track geometry, the wheel/track interface, average load per wheel, rotation drive method, etc. We discuss key considerations for the development of future suspension and rotation systems for ELT enclosures.
The GMT-Consortium Large Earth Finder (G-CLEF) is a fiber fed, optical echelle spectrograph that has been selected as
a first light instrument for the Giant Magellan Telescope (GMT) currently under construction at the Las Campanas
Observatory in Chile’s Atacama desert region. We designed G-CLEF as a general-purpose echelle spectrograph with
precision radial velocity (PRV) capability used for exoplanet detection. The radial velocity (RV) precision goal of GCLEF
is 10 cm/sec, necessary for detection of Earth-sized planets orbiting stars like our Sun in the habitable zone. This
goal imposes challenging stability requirements on the optical mounts and the overall spectrograph support structures.
Stability in instruments of this type is typically affected by changes in temperature, orientation, and air pressure as well
as vibrations caused by telescope tracking. For these reasons, we have chosen to enclose G-CLEF’s spectrograph in a
thermally insulated, vibration isolated vacuum chamber and place it at a gravity invariant location on GMT’s azimuth
platform. Additional design constraints posed by the GMT telescope include: a limited space envelope, a thermal
emission ceiling, and a maximum weight allowance. Other factors, such as manufacturability, serviceability, available
technology and budget are also significant design drivers. All of the previously listed considerations must be managed
while ensuring that performance requirements are achieved.
In this paper, we discuss the design of G-CLEF’s optical mounts and support structures including technical choices made
to minimize the system’s sensitivity to thermal gradients. A more general treatment of the properties of G-CLEF can be
found elsewhere in these proceedings1. We discuss the design of the vacuum chamber which houses the irregularly
shaped optical bench and optics while conforming to a challenging space envelope on GMT’s azimuth platform. We also
discuss the design of G-CLEF’s insulated enclosure and thermal control systems which maintain the spectrograph at
milli-Kelvin level stability while simultaneously limiting the maximum thermal emission into the telescope dome
environment. Finally, we discuss G-CLEF’s front-end assembly and fiber-feed system as well as other interface
challenges presented by the telescope, enclosure and neighboring instrumentation.
The Multi-Object Broadband Imaging Echellette (MOBIE) is the seeing-limited, visible-wavelength imaging multiobject
spectrograph (MOS) planned for first-light use on the Thirty Meter Telescope (TMT). The MOBIE project to
date has been a collaboration lead by UC Observatories (CA), and including the UH Institute for Astronomy (HI), and
the NAOJ (Tokyo, Japan). The current MOBIE optical design provides two color channels, spanning the 310–550nm
and 550-1000nm passbands, and a combination of reflection gratings, prisms, and mirrors to enable direct imaging and
three spectroscopic modes with resolutions (λ/triangle λ) of roughly 1000, 3000, and 8000 in both color channels, across a field of view that ranges from roughly 8x3 arcmin to 3x3 arcmin, depending on resolution mode. The conceptual design phase for the MOBIE instrument has been underway since 2008 and is expected to end in 2015. We report here on developments since 2010, including assembly of the current project team, instrument and camera optical designs,
instrument control systems, atmospheric dispersion corrector, slit-mask exchange systems, collimator, dichroic and fold
optics, dispersing and cross-dispersing optics, refracting cameras, shutters, filter exchange systems, science detector
systems, and instrument structures.
The GMT-Consortium Large Earth Finder (G-CLEF) is an optical-band echelle spectrograph that has been selected as
the first light instrument for the Giant Magellan Telescope (GMT). G-CLEF is a general-purpose, high dispersion
spectrograph that is fiber fed and capable of extremely precise radial velocity measurements. The G-CLEF Concept
Design (CoD) was selected in Spring 2013. Since then, G-CLEF has undergone science requirements and instrument
requirements reviews and will be the subject of a preliminary design review (PDR) in March 2015. Since CoD review
(CoDR), the overall G-CLEF design has evolved significantly as we have optimized the constituent designs of the major
subsystems, i.e. the fiber system, the telescope interface, the calibration system and the spectrograph itself. These
modifications have been made to enhance G-CLEF’s capability to address frontier science problems, as well as to
respond to the evolution of the GMT itself and developments in the technical landscape. G-CLEF has been designed by
applying rigorous systems engineering methodology to flow Level 1 Scientific Objectives to Level 2 Observational
Requirements and thence to Level 3 and Level 4. The rigorous systems approach applied to G-CLEF establishes a well
defined science requirements framework for the engineering design. By adopting this formalism, we may flexibly update
and analyze the capability of G-CLEF to respond to new scientific discoveries as we move toward first light. G-CLEF
will exploit numerous technological advances and features of the GMT itself to deliver an efficient, high performance instrument, e.g. exploiting the adaptive optics secondary system to increase both throughput and radial velocity
The Giant Magellan Telescope (GMT) is a 25.4-m diameter, optical/infrared telescope that is being built by an international consortium of universities and research institutions as one of the next generation of Extremely Large Telescopes. The primary mirror of GMT consists of seven 8.4 m borosilicate honeycomb mirror segments that are optically conjugate to seven corresponding segments in the Gregorian secondary mirror. Fabrication is complete for one primary mirror segment and is underway for the next two. The final focal ratio of the telescope is f/8.2, so that the focal plane has an image scale of 1.02 arcsec/mm. GMT will be commissioned using a fast-steering secondary mirror assembly comprised of conventional, rigid segments to provide seeing-limited observations. A secondary mirror with fully adaptive segments will be used in standard operation to additionally enable ground-layer and diffraction-limited adaptive optics. In the seeing limited mode, GMT will provide a 10 arcmin field of view without field correction. A 20 arcmin field of view will be obtained using a wide-field corrector and atmospheric dispersion compensator. The project has recently completed a series of sub-system and system-level preliminary design reviews and is currently preparing to move into the construction phase. This paper summarizes the technical development of the GMT sub-systems and the current status of the GMT project.
The preliminary design of the 25 m Giant Magellan Telescope (GMT) has been completed. This paper describes the design of the optics, structure and mechanisms, together with the rationales that lead to the current design. Analyses that were conducted to verify structure and optical performance are summarized. Science instruments will be mounted within the telescope structure. A common instrument de-rotator is provided to compensate for field rotation caused by the alt-az tracking of the telescope. The various instrument stations and provisions for mounting instruments are described. Post-PDR development plans for the telescope are presented.
We describe the Michigan/Magellan Fiber System (M2FS) under construction for use on the Magellan/Clay telescope.
M2FS consists of four primary components including: (1) A fiber-fed double spectrograph (MSPec) in which each
spectrograph is fed by 128 fibers (for a total multiplexing factor of 256) and each is optimized in to operate from 370-
950 nm; (2) A fiber mounting system (MFib) that supports the fibers and fiber plug plates at the telescope f/11 Nasmyth
focal surface and organizes the fibers into ‘shoes’ that are used to place the fibers at the image surface of the MSpec
spectrographs;, (3) A new wide-field corrector (WFC) that produces high-quality images over a 30 arcmin diameter
field; (4) A unit (MCal) mounted near the telescope secondary that provides wavelength and continuum calibration and
that supports a key component in a novel automated fiber identification system. We describe the opto-mechanical
properties of M2FS, its modes of operation, and its anticipated performance, as well as potential upgrades including the
development of a robotic fiber positioner and an atmospheric dispersion corrector. We describe how the M2FS design
could serve as the basis of a powerful wide-field, massively multiplexed spectroscopic survey facility.
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.
The Multi-Object Broadband Imaging Echellette (MOBIE) is the seeing-limited, optical spectrograph planned for the
first generation of Thirty Meter Telescope (TMT) instruments<sup>1</sup>. An end-to-end stray light analysis of the full optical path
(telescope to detector array) has been undertaken as a first step towards validating the design concept with regard to stray
light requirements. The geometric, stray light model includes the TMT Calotte-style dome structure, telescope optics,
telescope support structures, and the MOBIE instrument itself. The stray light calculations, including assumptions,
methodology, and conclusions, are described. Particular emphasis is placed on the stray light contributions from the
telescope, atmospheric dispersion corrector, and spectrograph optics. Recommendations for stray light controls internal
to the MOBIE instrument are discussed.
The Multi-Object Broadband Imaging Echellette (MOBIE) is the seeing-limited, wide-field multi-object optical imaging
spectrograph planned for first-light operation on the Thirty Meter Telescope (TMT). Following the completion of a
feasibility study and requirements review in December 2008, the MOBIE instrument project, based at the University of
California Observatories (UCO) on the UC Santa Cruz campus, entered a conceptual design phase. In this paper, we
describe the latest developments in the instrument optical design, and progress in the conceptual design of the optomechanical
and mechanical elements for the instrument.
We describe the construction and commissioning of FIRE, a new 0.8-2.5μm echelle spectrometer for the Magellan/
Baade 6.5 meter telescope. FIRE delivers continuous spectra over its full bandpass with nominal spectral
resolution R = 6000. Additionally it offers a longslit mode dispersed by the prisms alone, covering the full z to
K bands at R ~ 350. FIRE was installed at Magellan in March 2010 and is now performing shared-risk science
observations. It is delivering sharp image quality and its throughput is sufficient to allow early observations of
high redshift quasars and faint brown dwarfs. This paper outlines several of the new or unique design choices
we employed in FIRE's construction, as well as early returns from its on-sky performance.
The Dark Energy Survey (DES) will produce high quality images covering over 5000 square degrees of the sky,
with precise photometric redshifts between z = 0.2 to z = 1.3, using g, r, i, z and Y filters. The Dark Energy
Camera (DECam), under construction for this survey, consists of wide field corrector optics and a CCD detector
array that will give a 2.2 square degree field of view. It will be placed at the prime focus of the Blanco 4-meter
telescope at the Cerro Tololo Inter-American Observatory in Chile. The Optical Science Laboratory (OSL) at
University College London (UCL) is undertaking the alignment of the five lenses in the imaging system. These
lenses range in diameter from 0.60 - 0.98 meters. The lenses must be held within tight tolerance limits in order
to meet the DES science requirements. The tolerances are especially driven by the accuracy in the measurement
of the weak lensing signal. This paper details the design for the cells that will hold the lenses and the alignment
procedure for the mounting of the lenses and cells. Also presented is the expected static shear distortion pattern
that will be generated and the impact of this pattern on the weak lensing signal measurement.
Construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005 and since then
it has been in intensive commissioning. This has now almost been completed except for the telescope's image quality
which shows optical aberrations, chiefly a focus gradient across the focal plane, along with astigmatism and other less
significant aberrations. This paper describes the optical systems engineering investigation that has been conducted since
early 2006 to diagnose the problem. A rigorous approach has been followed which has entailed breaking down the
system into the major sub-systems and subjecting them to testing on an individual basis. Significant progress has been
achieved with many components of the optical system shown to be operating correctly. The fault has been isolated to a
major optical sub-system. We present the results obtained so far, and discuss what remains to be done.
The Magellan Echellette (MagE) spectrograph is a single-object optical echellette spectrograph for the
Magellan Clay telescope. MagE has been designed to have high throughput in the blue; the peak
throughput is 22% at 5600 Å including the telescope. The wavelength coverage includes the entire
optical window (3100 Å - 1 μm). The spectral resolution for a 1" slit is R~4100. MagE is a very
simple spectrograph with only four moving parts, prism cross-dispersion, and a vacuum Schmidt
camera. The instrument saw first light in November 2007 and is now routinely taking science
We describe a preliminary optical design for a multi-object, wide-field, optical echellette spectrograph that is intended to
serve a broad range of science. It will produce low-resolution, single-order spectra for survey-mode programs targeting
as many objects as possible and also moderate-resolution, multiple-order spectra for a reduced number of targets. The
design uses all refracting optics. The first optical element of the spectrograph is a wide-field corrector for the telescope
that causes the chief rays to be perpendicular to the focal plane. The collimator, which has been designed on-axis, can
then be duplicated to target multiple, off-axis fields in a multiple-barrel configuration. The collimator optics include an
achromatic field lens group that forms a sharp pupil over the full optical band-pass (320-1000 nm), followed by a
dichroic which splits the beam into a red and a blue channel. All remaining optical elements of the collimator, the
gratings, the cameras, and the detectors are then optimized for red or blue wavelengths. Both red and blue channels of
each beam of the spectrograph use reflection gratings to produce either a single-order spectrum at resolutions around
R=λ/Δλ=1000 or a five-order, R>5000 echellette spectrum with prism cross-dispersion. Both modes can target objects
anywhere in the collimated field of view. A direct imaging mode will also be provided.
FIRE (the Folded-port InfraRed Echellette) is a prism cross-dispersed infrared spectrometer, designed to deliver singleobject
R=6000 spectra over the 0.8-2.5 micron range, simultaneously. It will be installed at one of the auxiliary
Nasmyth foci of the Magellan 6.5-meter telescopes. FIRE employs a network of ZnSe and Infrasil prisms, coupled with
an R1 reflection grating, to image 21 diffraction orders onto a 2048 × 2048, HAWAII-2RG focal plane array.
Optionally, a user-controlled turret may be rotated to replace the reflection grating with a mirror, resulting in a singleorder,
longslit spectrum with R ~ 1000. A separate, cold infrared sensor will be used for object acquisition and guiding.
Both detectors will be controlled by cryogenically mounted SIDECAR ASICs. The availability of low-noise detectors
motivates our choice of spectral resolution, which was expressly optimized for Magellan by balancing the scientific
demand for increased R with practical limits on exposure times (taking into account statistics on seeing conditions).
This contribution describes that analysis, as well as FIRE's optical and opto-mechanical design, and the design and
implementation of cryogenic mechanisms. Finally, we will discuss our data-flow model, and outline strategies we are
putting in place to facilitate data reduction and analysis.
The DECam instrument, for the 4m Blanco telescope at CTIO, is a 5 lens element wide field camera giving a 2.2 degree
diameter field of view. The lenses are large, with the biggest being 980mm in diameter, and this poses challenges in
mounting and alignment. This paper reports the status of the production of the optics for the DECam wide field imager
Also presented are the design and finite element modelling of the cell design for the 5 lenses of the imager along with the
proposed alignment process.
We describe the Dark Energy Camera (DECam), which will be the primary instrument used in the Dark Energy Survey.
DECam will be a 3 sq. deg. mosaic camera mounted at the prime focus of the Blanco 4m telescope at the Cerro-Tololo
International Observatory (CTIO). DECam includes a large mosaic CCD focal plane, a five element optical corrector,
five filters (g,r,i,z,Y), and the associated infrastructure for operation in the prime focus cage. The focal plane consists of
62 2K x 4K CCD modules (0.27"/pixel) arranged in a hexagon inscribed within the roughly 2.2 degree diameter field of
view. The CCDs will be 250 micron thick fully-depleted CCDs that have been developed at the Lawrence Berkeley
National Laboratory (LBNL). Production of the CCDs and fabrication of the optics, mechanical structure, mechanisms,
and control system for DECam are underway; delivery of the instrument to CTIO is scheduled for 2010.
We describe a five element corrector for the prime focus of the 4 meter Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile that will be used in conjunction with a new mosaic CCD camera as part of the proposed Dark Energy Survey (DES). The corrector is designed to provide a flat focal plane and good images in the SDSS <i>g</i>, <i>r</i>, <i>i</i>, and <i>z</i> filters. We describe the performance in conjunction with the scientific requirements of the DES, particularly with regard to ghosting and weak-lensing point spread function (PSF) calibration.
Bigelow & Dressler<sup>1</sup> reported on the design and construction of IMACS - the Inamori-Magellan Areal Camera and Spectrograph. IMACS was installed on the Magellan-Baade 6.5-m telescope at the Carnegie Institution's Las Campanas Observatory in Chile in August, 2003, and was phased into regular operation in the remaining months of that year (Osip et al<sup>2</sup>). IMACS is now the most-used instrument on the Baade telescope, accounting for 63% of the nights available for astronomy in the 2005 observing year.
IMACS has two basic operating modes. A single 6-inch beam refractive collimator feeds either (1) an f/4 all-spherical refractive camera delivering 0.11 arcsec/pixel, or (2) a double-asphere refractive camera with oil-coupled multiplets producing a scale of 0.20 arcsec/pixel. The detector for both foci is an 8K x 8K mosaic camera of 8 SITe 2K x 4K 15 μ CCDs. The collimator and f/4 camera have performed to design specifications and have delivered 0.45 arcsec images across the 15 arcmin square field. The f/2 camera has delivered images of 0.55 to 0.65 arcsec across its 27 arcmin diameter field in excellent seeing (FWHM ~ 0.40 arcsec). The f/4 camera uses 6-inch reflecting gratings to obtain spectroscopy at multiple resolutions ranging from R=1350-9375; the f/2 camera uses three 6-inch grisms to achieve resolutions of R=450, 600, and 900 over its larger field. We routinely cut hundreds of slits in 30-inch diameter, stainless steel, spherical-shell slitmasks with a commercial laser system. Alignment procedures for observing are simple and efficient, typically requiring 5-10 minutes per set-up.
IMACS - an unusually versatile instrument - includes an IFU built by Durham University with two 5" x 8" (f/2) or 4" x 7" (f/4) apertures, each sampled by 1000 optical fibers. A Multi-Object Echelle mode, which can obtain 10-15 full wavelength R=20000 spectra, has been fully tested and has now started regular operation. The Maryland-Magellan Tunable Filter (MMTF) has been lab tested and will be commissioned in June 2006. In early 2007, Gladder's Image-Slicing Multislit Option (GISMO) will be ready for testing, and a second Mosaic CCD camera - which will simplify operations, increase sensitivity, and allow rapid access to both f/2 and f/4 modes - is under construction.
We report on the design challenges posed and met by the variety of operating modes and stringent performance requirements. We describe some issues encountered in the past two years in bringing such a complex, multi-mode instrument to the Magellan Observatory.
Mission requirements, the baseline design, and optical systems budgets for the SuperNova/Acceleration Probe (SNAP) telescope are presented. SNAP is a proposed space-based experiment designed to study dark energy and alternate explanations of the acceleration of the universe’s expansion by performing a series of complementary systematics-controlled astrophysical measurements. The goals of the mission are a Type Ia supernova Hubble diagram and a wide-field weak gravitational lensing survey. A 2m widefield three-mirror telescope feeds a focal plane consisting of 36 CCDs and 36 HgCdTe detectors and a high-efficiency, low resolution integral field spectrograph. Details of the maturing optical system, with emphasis on structural stability during terrestrial testing as well as expected environments during operations at L2 are discussed. The overall stray light mitigation system, including illuminated surfaces and visible objects are also presented.
The Inamori Magellan Areal Camera and Spectrograph (IMACS) will soon be one of the three first-generation instruments for the Magellan 6.5m telescopes. This instrument drove the specification and design of the f/11 Gregorian focus on Magellan, which it uses to feed an all-spherical, refracting wide-field collimator with a 30 arcmin field of view. Two Epps cameras are used to re-image the field of view for imaging and spectroscopy. The aspheric, f/2 ("short") camera images a field of 27 x 27 arcmin at 0.2 arcsec/pixel, and produces 0.32 arcsec images averaged over all field positions across the 0.39 -1.05 micron bandpass. The all-spherical f/4 ("long") camera images a field 15 x 15 arcmin at 0.11 arcsec/pixel, and produces 0.16 arcsec images averaged over all field positions across the 0.365 - 1.0 micron bandpass. This paper describes the final specifications for the multiple spectrographic and imaging modes, and provides a status report on the current state of the instrument project.
The Inamori-Magellan Areal Camera and Spectrograph (IMACS) features a 8K x 8K, 6.7 Megapixel detector system, which is mounted in a cryogenic vacuum vessel with a combination of features that are unique among the current generation of astronomical multi-detector array systems. Closed-cycle coolers, commercial stages for flexure compensation, flexure control detectors, array focus control, composite thermal isolation truss and other features are described.
The Southern African Large Telescope (SALT) is a 10-m class telescope presently under construction at Sutherland in South Africa. It is designed along the lines of the Hobby-Eberly Telescope (HET) at McDonald Observatory in West Texas. SALTICAM will be the Acquisition Camera and simple Science Imager (ACSI) for this telescope. It will also function as the Verification Instrument (VI) to check the performance of the telescope during commissioning.
In VI mode, SALTICAM will comprise a filter unit, shutter and cryostat with a 2x1 mosaic of 2k x 4k x 15 micron pixel CCDs. It will be mounted at the f/4.2 corrected prime focus of the telescope. In ACSI mode it will be fed by a folding flat located close to the exit pupil of the telescope. ACSI mode will have the same functional components as VI mode but it will in addition be garnished with focal conversion lenses to re-image the corrected prime focal plane at f/2. The lenses will be made from UV transmitting crystals as the wavelength range for which the instrument is designed will span 320 to 950 nm.
In addition to acting as Verification Instrument and Acquisition Camera, SALTICAM will perform simple science imaging in support of other instruments, but will also have a high time resolution capability which is not widely available on large telescopes.
This paper will describe the design of the instrument, emphasizing features of particular interest.
The Inamori-Magellan Areal Camera and Spectrograph is nearing completion. This reimaging spectrograph will have fields of view of 15 arcmin and 27 arcmin in its relecting grating and grism spectrographic modes, respectively, the largest such areas available on one of the new generation of large optical-IR ground-based telescopes. In addition to wide field imaging and a range of low- to medium-resolution spectroscopic modes, IMACS will have a 2 × 1000 fiber-fed integral field unit built by Durham University, an ecellette mode, and the potential for a full-field tunable filter. We review some of the planned science programs for IMACS, ranging from spectroscopy of stars in the Galactic halo and nearby dwarf spheroidal galaxies, the search for stars between galaxies, internal kinematics in normal galaxies and AGN, and the evolution of high redshift galaxies and galaxy clusters.
There are considerable potential advantages in throughput, polarization and IR emissivity of performing the adaptive correction at the telescope secondary. However, reliability and safety are key issues. This paper explores a sound engineering approach which is rugged, safe and provides a level of redundancy. A 7-actuator demonstrator is described, together with first results from laboratory testing.
The Echellette Spectrograph and Imager (ESI) is one of several second-generation instruments for the Keck telescopes. The motivation for the f/15 Cassegrain-mounted instrument has been to provide a versatile, extremely efficient, and stable system for faint object spectroscopy and imaging, on a comparatively limited schedule and budget. In keeping with these goals, a space-frame instrument structure has been designed, analyzed, and fabricated. The mainframe structure provides the mechanical interface between the telescope and instrument, support points for all the optical, mechanical, and electronic sub-systems, and provides a rigid base for the active- collimator flexure control system. The fundamental concepts and motivation for using a space-frame are discussed, and their application to the design, analysis, and fabrication of the ESI structure is presented.
The Inamori Magellan Areal Camera and Spectrograph (IMACS) will be one of three first-generation instruments for the Magellan 6.5 m telescopes. It will be installed at the f/11 (Gregorian) Nasmyth focus. This instrument drove the specification and design of the f/11 configuration, which it uses to feed an all-spherical, wide-field collimator. The combination of the Gregorian secondary and refracting collimator lead to 0.2 arc-sec images over a 17 arc-min field with an f/2.66 camera, and 0.4 arc-sec images over a 27 arc- min field with an f/1.49 camera. This paper describes the preliminary specifications for the multiple spectrographic and imaging modes, the optical layout of the instrument and Epps cameras, and strategies for the design and fabrication of the instrument.
The Echellette Spectrograph and Imager (ESI) is being built at UCO/Lick Observatory for the Cassegrain focus of the Keck II telescope. The collimator mirror is optimally constrained by a space-frame structure. It will be actively moved to provide the focus and flexure (tip and tilt) control for the instrument. Careful attention to space-frame geometry has simplified the mechanical design. Analytical and Finite Element Analysis (FEA) are presented to demonstrate how a simple but very stiff structure is used to provide support, flexure control, and focus.
We have previously proposed the use of commercially available actuators and displacement sensors for use in the control of large deformable secondary mirrors. We have identified the magneto-strictive (MS) actuator as a promising candidate based on manufacturer's specifications for stroke, power consumption, and service life. We have identified both capacitive and eddy-current displacement sensors as possible choices for completing the required control loop around the actuators. The purpose of the tests described here was to characterize the performance of a MS actuator in terms of hysteresis, linearity, power consumption, heat dissipation, and frequency response, and to confirm the manufacturer's specifications for longevity. The purpose of the sensor testing was to compare their performance in terms of frequency response and packaging constraints. Results of the testing program are presented.
We define the stability requirements for a high-resolution spectrograph, then show how these can be met at Cassegrain by modern materials, mechanism design, thermal control, and passive and active compensation for structural flexure. We consider the optimization of the information throughput of the spectrograph, in terms of slit-throughput, with the superb imaging performance of modern large telescopes and sites, new developments in image slicers, the prospects for adaptive-optics feeds for spectrographs, and the internal transmission of the optics. We consider in detail the requirements of, and solutions for, high resolution spectrographs for two large telescope projects - the 6.5 m MMT conversion and the two Gemini 8 m telescopes.
We describe the high resolution echelle spectrometer (HIRES) now in operation on the Keck Telescope. HIRES, which is permanently located at a Nasmyth focus, is a standard in-plane echelle spectrometer with grating post dispersion. The collimated beam diameter is 12', and the echelle is a 1 x 3 mosaic, 12' by 48' in total size, of 52.6 gr mm<SUP>MIN1</SUP>, R-2.8 echelles. The cross disperser is a 2 x 1 mosaic, 24' by 16 ' in size. The camera is of a unique new design: a large (30' aperture) f/1.0, all spherical, all fused silica, catadioptric system with superachromatic performance. It spans the entire chromatic range from 0.3 (mu) to beyond 1.1 (mu) , delivering 12.6-micron (rms) images, averaged over all colors and field angles, without refocus. The detector is a thinned, backside-illuminated, Tektronix 2048 x 2048 CCD with 24-micron pixels, which spans the spectral region from 0.3 (mu) to 1.1 (mu) with very high overall quantum efficiency. The limiting spectral resolution of HIRES is 67,000 with the present CCD pixel size. The overall 'throughput' (resolution x slit width) product achieved by HIRES is 39,000 arcseconds. Peak overall efficiency for the spectrograph (not including telescope and slit losses) is 13% at 6000 angstrom. Some first-light science activities, including quasar absorption line spectra, beryllium abundances in metal-poor stars, lithium abundances in brown-dwarf candidates, and asteroseismology are discussed.
The design of a proposed 1-m diameter deformable secondary mirror is described, with emphasis on optimization of each component for adaptive optical correction. Finite element analyses are presented which investigate the ability of the mirror to match low-order Zernike polynomials. Wavefront data from a 1-m telescope are used to examine the ability of the mirror to fit atmospherically distorted wavefronts. Finally, mirror surface FEA results from the atmospheric wavefronts are Fourier transformed to predict pre- and post-correction image quality.
The finite element analyses of two large lenses for the Keck Telescope High Resolution Echelle Spectrograph are described. The two lenses, one simple lens, and one meniscus, are of fused silica and are approximately 800 mm (30 in.) in diameter. The purpose of the analyses is to determine the deformations of each optic under its own weight, and to identify the simplest, most cost effective mounting cell that will satisfy the optical requirements. Two common radial supports are analyzed, including varieties of hard point and band type mountings. Several types of axial supports are examined including simple three-point mounts, ring mounts, and static deformation mounts. A parametric finite element input routine is described, whereby a solid model and finite element mesh are automatically generated, given the lens diameter, central thickness, and surface radii of curvature. Deformation predictions from the models are compared with theoretical calculations, interferometric testing, and precision profilometry.