The Giant Magellan Telescope adaptive optics system will be an integral part of the telescope, providing laser guide star
generation, wavefront sensing, and wavefront correction to most of the currently envisioned instruments. The system
will provide three observing modes: Natural Guidestar AO (NGSAO), Laser Tomography AO (LTAO), and Ground
Layer AO (GLAO).
Every AO observing mode will use the telescope’s segmented adaptive secondary mirror to deliver a corrected beam
directly to the instruments. High-order wavefront sensing for the NGSAO and LTAO modes is provided by a set of
wavefront sensors replicated for each instrument and fed by visible light reflected off the cryostat window. An infrared
natural guidestar wavefront sensor with open-loop AO correction is also required to sense tip-tilt, focus, segment piston,
and dynamic calibration errors in the LTAO mode. GLAO mode wavefront sensing is provided by laser guidestars over
a ~5 arcminute field of view, and natural guidestars over wider fields. A laser guidestar facility will project 120 W of
589 nm laser light in 6 beacons from the periphery of the primary mirror. An off-axis phasing camera and primary and
secondary mirror metrology systems will ensure that the telescope optics remain phased.
We describe the system requirements, overall architecture, and innovative solutions found to the challenges presented by high-order AO on a segmented extremely large telescope. Further details may be found in specific papers on each of the observing modes and major subsystems.
The 25 m Giant Magellan Telescope consists of seven circular 8.4 m primary mirror segments with matching
segmentation of the Gregorian secondary mirror. Achieving the diffraction limit in the adaptive optics observing modes
will require equalizing the optical path between pairs of segments to a small fraction of the observing wavelength. This
is complicated by the fact that primary mirror segments are separated by up to 40 cm, and composed of borosilicate
glass. The phasing system therefore includes both edge sensors to sense high-frequency disturbances, and wavefront
sensors to measure their long-term drift and sense atmosphere-induced segment piston errors.
The major subsystems include a laser metrology system monitoring the primary mirror segments, capacitive edge
sensors between secondary mirror segments, a phasing camera with a wide capture range, and an additional sensitive
optical piston sensor incorporated into each AO instrument. We describe in this paper the overall phasing strategy,
controls scheme, and the expected performance of the system with respect to the overall adaptive optics error budget.
Further details may be found in specific papers on each of the subsystems.
Achieving the diffraction limit with the adaptive optics system of the 25m Giant Magellan Telescope will require that
the 7 pairs of mirror segments be in phase. Phasing the GMT is made difficult because of the 30-40cm gaps between the
primary mirror segments. These large gaps result in atmospheric induced phase errors making optical phasing difficult
at visible wavelengths. The large gaps between the borosilicate mirror segments also make an edge sensing system
prone to thermally induced instability. We describe an optical method that uses twelve 1.5-m square subapertures that
span the segment boundaries. The light from each subaperture is mapped onto a MEMS mirror segment and then a
lenslet array which are used to stabilize the atmospherically induced image motion. Centroids for stabilization are
measured at 700nm. The piston error is measured from the fringes visible in each of the 12 stabilized images at 2.2
microns. By dispersing the fringes we can resolve 2π phase ambiguities. We are constructing a prototype camera to be
deployed at the 6.5m Magellan Clay telescope.
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.
ARGOS is the Laser Guide Star adaptive optics system for the Large Binocular Telescope. Aiming for a wide field
adaptive optics correction, ARGOS will equip both sides of LBT with a multi laser beacon system and corresponding
wavefront sensors, driving LBT's adaptive secondary mirrors. Utilizing high power pulsed green lasers the artificial
beacons are generated via Rayleigh scattering in earth's atmosphere. ARGOS will project a set of three guide stars above
each of LBT's mirrors in a wide constellation. The returning scattered light, sensitive particular to the turbulence close to
ground, is detected in a gated wavefront sensor system. Measuring and correcting the ground layers of the optical
distortions enables ARGOS to achieve a correction over a very wide field of view. Taking advantage of this wide field
correction, the science that can be done with the multi object spectrographs LUCIFER will be boosted by higher spatial
resolution and strongly enhanced flux for spectroscopy. Apart from the wide field correction ARGOS delivers in its
ground layer mode, we foresee a diffraction limited operation with a hybrid Sodium laser Rayleigh beacon combination.
ARGOS is an innovative multi-star adaptive optics system being built for use with LUCIFER on the Large Binocular
Telescope (LBT). LUCIFER is a wide field imager and multi-object spectrograph. Using a constellation of laser guide
stars permits PSF correction over a wide field in exchange for a relatively small sacrifice in achievable correction. The
laser constellation consists of three stars per each of the two eyes of the LBT. The stars are nominally positioned on a circle
2' in radius, but each star can be moved by upto 0.5' in any direction. Nd:YAG (SHG) lasers from InnoLas Laser GmbH
are used to create the green (532nm) laser stars, and have an output above 18 W each at the planned pulsing frequency of
10kHz. The lasers are launched using a 40cm telescope and focused at a height of 12 km. The laser system is designed
to be optically simple yet configurable. It also provisions for a central sodium laser to be installed later. We detail the
characteristics of the laser system and the current state of its development.
We present the integration of a low dark current extended wavelength (2.3μm cutoff) InGaAs array into the
Cornell-Massachusetts Slit Spectrograph (CorMASS) spectrograph. The InGaAs array was fabricated onto a SB-
206 512×512 readout integrated circuit (ROIC) by Goodrich/Sensors Unlimited and subsequently went through a
series of laboratory characterization tests at the University of Virginia demonstrating dark current performance
of better than 10 e-/s. The InGaAs array is adapted for use with the CorMASS to verify its performance in a
proven astronomical instrument, and for eventual deployment to a telescope to test stability and performance.
Recent development of low dark current 2.34 μm-cutoff InGaAs material has resulted in the successful construction of a hybrid focal plane array built on the SB-206 512×512 format astronomical quality Read Out Integrated Circuit(ROIC). This contribution reports on the verification of the quality of the InGaAs material as well as the essential characteristics and performance of the hybrid focal plane array. The results of the investigation indicate that the dark current levels surpass the requirements for ground-based broadband and narrowband imaging as
well as for low resolution spectroscopy in the astronomical H and Ks bands.
A small research grant from the AAS has enabled the addition of a pair of MgF2 Wollaston prism polarization analyzers to the Fan Mountain Near Infrared Camera (FanCam). FanCam is a HAWAII-1 (1K × 1K HgCdTe) near infrared camera attached to the 0.8m Cassegrain reflector at Fan Mountain Observatory, 15 miles south of Charlottesville, Virginia. It images an 8.5 × 8.5 arcmin field of view with 0.51 arcsec pixels through a variety of broad band and narrow band filters, including JHKs, Brγ, and H2. The polarizers are mounted in one of the two camera filter wheels in the cold collimated beam near the re-imaged pupil and are oriented such that the direction of the separation of the split polarized images from one prism is rotated 45° relative to that from the other prism. The linear Stokes parameters of uncrowded point sources over a 7.5 × 7.5arcmin field of view may be measured by aperture photometry of pairs of images acquired through the two prisms. Initial obervations of polarized and unpolarized standard stars show that measurements of the degree of polarization are repeatable to within a few tenths of a percent, consistent with photon counting statistics. More standard star observations will be necessary to determine precisely the instrumental polarization and position angle offsets, but they appear to be stable and reasonably small.
We report the results of a program to mitigate defect induced (tunneling) dark current which arises from lattice
mismatch between In0.82Ga0.18As 'extended wavelength' detector material and the InP substrate upon which
it is grown. Our goal is to produce material suitable for ground-based broadband astronomical observation by
achieving a dark current level in individual 25x25μm array pixels which is less than the atmospheric airglow
and telescope thermal emission in the astronomical H (1.50-1.80 μm) and Ks (2.00-2.32 μm) bands. We have
cryogenically tested multiple growths of candidate materials, packaged as both individual diodes and focal plane
arrays, supplied by Sensors Unlimited, Inc. (SU). Results indicate dark current levels, in the current generation
of array materials, surpassing the requirements for broadband imaging, and with the potential to be used for
narrow band imaging and low-resolution spectroscopy.
We describe the optical and mechanical design of a simple hand-held near infrared spectrograph constructed
to produce observations of the spectrum of scrambled light from the Earth from aboard the International Space
Station. Observing the Earth in this manner simulates the changing perspective on an extra-solar terrestrial
planet observed as a point source by the Terrestrial Planet Finder. A Sensors Unlimited, Inc. SU320-M
InGaAs(0.86 - 1.72μm) camera detects the dispersed spectrum and outputs NTSC video to be recorded and
also permits frame grabbing. One of the three copies of the instrument is currently aboard the International
Space Station. The optical and mechanical design was conceived and executed by graduate and undergraduate
students at the University of Virginia.