This PDF file contains the front matter associated with SPIE Proceedings Volume 8149, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
The Large Binocular Telescope (LBT) is a unique telescope featuring two co-mounted optical trains with 8.4m primary
mirrors. The telescope Adaptive Optics (AO) system uses two innovative key components, namely an adaptive
secondary mirror with 672 actuators and a high-order pyramid wave-front sensor. During the on-sky commissioning such
a system reached performances never achieved before on large ground-based optical telescopes. Images with 40mas
resolution and Strehl Ratios higher than 80% have been acquired in H band (1.6 μm). Such images showed a contrast as
high as 10-4. Based on these results, we compare the performances offered by a Natural Guide Star (NGS) system
upgraded with the state-of-the-art technology and those delivered by existing Laser Guide Star (LGS) systems. The
comparison, in terms of sky coverage and performances, suggests rethinking the current role ascribed to NGS and LGS
in the next generation of AO systems for the 8-10 meter class telescopes and Extremely Large Telescopes (ELTs).
Exoplanet imaging is driving a race to higher contrast imaging, both from earth and from space. Next-generation
instruments such as the Gemini Planet Imager (GPI) and SPHERE are designed to achieve contrast ratios of
10-6 - 10-7 this requires very good wavefront correction and coronagraphic control of diffraction. GPI is a
facility instrument, now in integration and test, with first light on the 8-m Gemini South telescope expected
by the middle of 2012. It combines a 1700 subaperture AO system using a MEMS deformable mirror, an
apodized-pupil Lyot coronagraph, a high-accuracy IR interferometric wavefront calibration system, and a nearinfrared
integral field spectrograph to allow detection and characterization of self-luminous extrasolar planets
at planet/star contrast ratios of 10-7. In this paper we will discuss the status of the integration and test now
taking place at the University of Santa Cruz California.
The Alignment and Phasing System (APS) is one of the key parts of the Thirty Meter Telescope (TMT) active optics system,
with the responsibility for evaluating and correcting the pre-adaptive optics wavefront delivered by the telescope. APS
is a high complexity system comprising a multi-channel wavefront-sensing instrument that produces as many as 250,000
discrete measurements and control software that delivers commands to about 12,000 active optics system actuators. The
APS software is designed to guide the instrument development and predict its performance via simulation and also ultimately
to serve the physical instrument itself. The software is built around a modeling tool we have developed called
TMTracer, which is tailored to the optical alignment of extremely large segmented telescopes. We will present the underlying
philosophy of TMTracer, as well as its architecture and sample simulation results that demonstrate its capabilities.
TNO is developing the Optical Tube Asssemblies (OTAs) for the ESO VLT Four Laser Guide Star Facility. The OTAs
are Galilean 20x beam expanders, expanding a Ø15 mm input beam to a steerable Ø300 mm output beam with a
wavefront quality requirement of 50 nm rms. The allowed defocus under the influence of the changing environmental air
temperature (0-15°C, -0.7°C/hr gradient) is only 0.2 waves. The thermal behaviour of the system has been analyzed by
combining optical, lumped mass and FE analyses. A design that is passively athermalized over a large temperature range
as well as under the influence of thermal gradients has been developed. Extensive thermal testing has shown the system
performs as required. This paper describes the design and test results.
In this paper we present the current status of control algorithm development for the Thirty Meter Telescope (TMT) Alignment
and Phasing System (APS).We discuss ways to address the main challenges inherent in the active control of extremely
large segmented telescopes: high complexity of the control problem, disentangling the aberrations on the primary, secondary
and tertiary mirrors, and the tight requirements for residual errors. We also present preliminary APS performance
estimates derived from simulations. In particular, our simulations show that the tomographic aberration disentanglement is
only marginally useful for TMT alignment.
Laser tomography capability using a multi laser guide star (LGS) system is being implemented at the 6.5 m MMT
telescope on Mt. Hopkins, AZ. The system uses five range-gated and dynamically refocused Rayleigh laser beacons to
sense the atmospheric wavefront aberration. Corrections are then applied to the wavefront using the 336-actuator
adaptive secondary mirror of the telescope. So far, the system has demonstrated successful control of ground-layer
aberration over a field of view substantially wider than is delivered by conventional adaptive optics. In this paper, we
report the latest results from this mode of operation, using for the first time a plate scale on our IR science camera that
samples the diffraction scale at the Nyquist limit. We also discuss findings for a reduction in the width of the on-axis
point-spread function from 1.07" to <0.2" in H band and present the progress achieved toward the implementation of
laser tomography. This will be attempted by means of a least squares reconstructor, which is obtained using
simultaneous measurements of the wavefronts from the LGS and an additional natural guide star.
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system uses advanced coronagraphic technique
for high contrast imaging of exoplanets and disks as close as 1 lambda/D from the host star. In addition
to unusual optics, achieving high contrast at this small angular separation requires a wavefront sensing and
control architecture which is optimized for exquisite control and calibration of low order aberrations. The
SCExAO system was thus designed to include the wavefront sensors required for bias-free high sensitivity and
high speed wavefront measurements. Information is combined from two infrared wavefront sensors and a fast
visible wavefront sensors to drive a single MEMS type deformable mirror mounted on a tip-tilt mount. The
wavefront sensing and control architecture is highly integrated with the coronagraph system.
In this paper we explain why a non-linear curvature wavefront sensor (nlCWFS) is more sensitive
than conventional wavefront sensors such as the Shack Hartmann wavefront sensor (SHWFS) and
the conventional curvature wavefront sensor (cCWFS) for sensing mV < 14 natural guide stars.
The non-linear approach builds on the successful curvature wavefront sensing concept but uses a
non-linear Gerchberg-Saxton (GS) phase diversity algorithm to reconstruct the wavefront. The nonlinear
reconstruction algorithm is an advantage for sensitivity but a challenge for fast computation.
The current speed is a factor of 10 to 100 times slower than needed for high performance groundbased
AO. We present a two step strategy to increase the speed of the algorithm. In the last
paper3 we presented laboratory results obtained with a monochromatic source, here we extend our
experiment to incorporate a broadband source. The sensitivity of the nlCWFS depends on the
ability to extract wavefront phase from diffraction limited speckles therefore it is essential that
the speckles do not suffer from chromatic aberration when used with a polychromatic source. We
discuss the design for the chromatic re-imaging optics, which through chromatic compensation,
allow us to obtain diffraction limited speckles in Fresnel propagated planes on either side of the
Since the realization of the twin Keck telescopes of 10-meter diameter built atop the Mauna Kea in Hawaii, the
technology of segmented mirrors has become a cornerstone for on-going projects of Extremely Large Telescopes (ELT).
Here the individual mirror segments should actually be phased together (i.e. reconstruct the surface of an ideal
continuous, giant mirror) within accuracies typically better than one tenth of the operating wavelength. This could be
achieved using existing Wavefront Sensors (WFS), but may also involve the development of alternative methods: in this
communication is described a new generation WFS operating in the image plane and able to sense differential piston
errors of the segments with residual uncertainties inferior to 25 nm by means of a phase-shifting technique. We describe
the principle of the method in both monochromatic and polychromatic light and present its achievable performance in
terms of limiting magnitude of the guide star in presence of various noise sources. It is emphasized that the technique is
also applicable for co-phasing sparse aperture interferometers, or more generally to any Adaptive Optics (AO) system
making use of image plane WFS evaluating telescope wavefront errors (WFE) in real time.
The Giant Magellan Telescope (GMT) will place seven primary mirror segments of 8.4 m diameter on a common mount
to form a single co-phased aperture of 25 m. 1High order adaptive optics (AO) using an adaptive secondary mirror that is
segmented in the same way as the primary will correct the telescope's imaging to the diffraction limit in the near
infrared. 2Critical to the performance of the telescope will be real-time correction of atmospherically-induced optical
path differences between the primary mirror segments. Measuring these errors is challenging because of the large gaps
between the segments, where the aberrated wavefront is not explicitly measured by the AO sensors, which are
approximately 30 cm even at their narrowest points.
In this paper we show that it will be feasible to estimate the path differences between the segments from the commands
sent to the adaptive secondary mirror while the AO is running in closed loop. These commands will be an approximate
representation of the open-loop atmospheric wavefronts. We have investigated the value of the approach with real-time
closed-loop deformable mirror command data from the first-light AO system now running on the Large Binocular
Telescope (LBT). 3,4The data are of very high quality and realistically capture the spatio-temporal behavior of the
wavefront. We use data from two nights to show that the GMT segment pathlength errors may be recovered to <25 nm
accuracy with a simple linear estimator. Additional simulations show similar performance, which, with high-order AO, is
quite adequate to maintain high Strehl ratio at near infrared wavelengths.
Carbon-fiber reinforced polymer (CFRP) composite is an attractive material for fabrication of optics due to its high
stiffness-to-weight ratio, robustness, zero coefficient of thermal expansion (CTE), and the ability to replicate multiple
optics from the same mandrel. We use 8 and 17 cm prototype CFRP thin-shell deformable mirrors to show that residual
CTE variation may be addressed with mounted actuators for a variety of mirror sizes. We present measurements of
surface quality at a range of temperatures characteristic of mountaintop observatories. For the 8 cm piece, the figure
error of the Al-coated reflective surface under best actuator correction is ~43 nm RMS. The 8 cm mirror has a low
surface error internal to the outer ring of actuators (17 nm RMS at 20°C and 33 nm RMS at -5°C). Surface roughness is
low (< 3 nm P-V) at a variety of temperatures. We present new figure quality measurements of the larger 17 cm mirror,
showing that the intra-actuator figure error internal to the outer ring of actuators (38 nm RMS surface with one-third the
actuator density of the 8 cm mirror) does not scale sharply with mirror diameter.
While the concept of wavefront decomposition is a foundation of active optics systems, the choice of basis functions for
mirror figure control is divided. The common functions are Zernike polynomials, ubiquitously used for wavefront
descriptions, and bending (also called minimum energy or vibration) modes which offer optimal performance. We
present a look at the comparative performance between the two approaches, and discuss an implementation approach
which seeks to combine much of the analytic and interface simplicity of Zernike polynomials with the improved
performance of bending modes.
Variation in density structure and altitude of mesospheric sodium impacts the performance of Adaptive Optics (AO). With large entrance apertures, Laser Guide Stars (LGS) are seen as cylinders with an intensity structure that reflects the density structure of the sodium layers. Such elongation spreads the sodium light over more WaveFront Sensor (WFS) pixels and reduces the signal to noise ratios. This effect is proportional to the square of the telescope diameter: 30 m for TMT and 42 m for E-ELT.
NA variability is examined and the resulting elongation analyzed for impact on WaveFront Error. TMT and E-ELT will be compared for methods being applied to mitigate LGS elongation to reduce resulting WaveFront Error.
Wavefront sensors, which use solid-state CCD or CMOS photosensors, are sources of errors in adaptive optic
systems. Inaccuracy in the detection of wavefront distortions introduces considerable errors into wavefront reconstruction
and leads to overall performance degradation of the adaptive optics system. The accuracy of wavefront
sensors is significantly affected by photosensor noise. Thus, it is crucial to formulate high-level photosensor models
that enable adaptive optic engineers to simulate realistic effects of noise from wavefront sensors. However,
the complexity of solid-state photosensors and multiple noise sources makes it difficult to formulate an adequate
model of the photosensor. Moreover, the characterisation of the simulated sensor and comparison with real
hardware is often incomplete due to lack of comprehensive standards and guidelines. Owe to these difficulties,
engineers work with oversimplified models of the wavefront sensors and consequently have imprecise numerical
The paper presents an approach for the modelling of noise sources for CCD and CMOS sensors that are used for
wavefront sensing in adaptive optics. Both dark and light noise such as fixed pattern noise, photon shot noise,
and read noises, as well as, charge-to-voltage noises are described. Procedures for characterisation of both light
and dark noises of the simulated photosensors are provided. Numerical simulation results of a photosensor for a
high-frame rate Shack-Hartmann wavefront sensor are presented.
A Shack-Hartmann (SH) wavefront sensor (WFS) is used in most modern adaptive optics systems where precision
and robustness of centroiding are important issues. The accuracy of the SH WFS depends not only on lenslet
quality but also on the measurement accuracy of centroids, especially in low-light conditions. In turn, accuracy
depends on light and dark noises that are inevitably present in solid-state photosensors. Using a comprehensive
mathematical model of the CMOS photosensor, the accuracy of the Shack-Hartmann wavefront sensor is assessed
and analysed for each type of noise.
In this paper, new results regarding the influence of different noise sources from a CMOS photosensor on centroiding
in Shack-Hartmann wavefront sensors are presented. For the numerical simulations, a comprehensive
mathematical model of photosensor's noise was formulated. The influences of light and dark noises as well as
pixelisation factor have been assessed. Analysis of the wavefront sensor's accuracy is provided. Results should
be of interest for further development of cost-effective wavefront sensors.
Large sky area multi-object fiber spectroscopy telescope (LAMOST) is an innovative reflecting Schmidt telescope. One
of its key technology is 4000 dual rotational fiber robot located in the focal plane. This article analyzes the calibration
requirements of the 4000 fiber robot. And then, proposes a fast calibration method in the complex field environment, and
discribes the specific process how to obtain positioning parameters of the fiber robot rapidly.
High powered guide star laser beams are a potential hazard for aircraft. Currently at the MMT telescope located on Mt.
Hopkins in Southern Arizona, five Rayleigh guide stars create a total of 25 W of power at 532 nm wavelength. The
ARGOS laser guide star for the Large Binocular Telescope (LBT) located on Mt. Graham in Southern Arizona will
generate six Rayleigh guide stars with a total of 108 W at 532 nm. We present an automated system for use at the MMT
and the LBT designed to detect aircraft and shutter the lasers when aircraft illumination is pending. The detection system
at the MMT uses a single wide-angle CCD camera mounted to the optical support structure of the telescope. The LBT
system employs two of the same CCD cameras, and an additional bore-sighted thermal infrared camera. The visible
cameras integrate frames for 0.5 s to produce streaks from anti-collision beacons required for all aircraft. The IR camera
serves as a backup and to protect unlighted aircraft. In each case, adjacent frames are compared using image processing
software to detect streaks and movement in the field. If an aircraft is detected, the position and projected trajectory are
calculated and compared to the position of the laser beams. If an aircraft illumination appears likely, the laser safety
shutter is closed and a message is sent to the laser operator. As a safety precaution, a heartbeat signal from the control
computer is required to keep the laser shutter open.
ARGOS, the laser-guided adaptive optics system for the Large Binocular Telescope (LBT), is now under construction at
the telescope. By correcting atmospheric turbulence close to the telescope, the system is designed to deliver high
resolution near infrared images over a field of 4 arc minute diameter. Each side of the LBT is being equipped with three
Rayleigh laser guide stars derived from six 18 W pulsed green lasers and projected into two triangular constellations
matching the size of the corrected field. The returning light is to be detected by wavefront sensors that are range gated
within the seeing-limited depth of focus of the telescope. Wavefront correction will be introduced by the telescope's
deformable secondary mirrors driven on the basis of the average wavefront errors computed from the respective guide
star constellation. Measured atmospheric turbulence profiles from the site lead us to expect that by compensating the
ground-layer turbulence, ARGOS will deliver median image quality of about 0.2 arc sec across the JHK bands. This will
be exploited by a pair of multi-object near-IR spectrographs, LUCIFER1 and LUCIFER2, with 4 arc minute field already
operating on the telescope. In future, ARGOS will also feed two interferometric imaging instruments, the LBT
Interferometer operating in the thermal infrared, and LINC-NIRVANA, operating at visible and near infrared
wavelengths. Together, these instruments will offer very broad spectral coverage at the diffraction limit of the LBT's
combined aperture, 23 m in size.