The Active optics, Guiding, and Wavefront Sensing system (AGWS), currently being designed by SAO, will use J-band dispersed fringe sensors (DFS) to phase the GMT to a fraction of an imaging wavelength. These phasing sensors will use off-axis guide stars to measure phase shifts at each of 12 segment boundaries. The fringes produced at each boundary will be dispersed in the perpendicular direction using an array of high-index doublet prisms. Inter-segment phase shifts will appear as tilts in the dispersed fringes, which can be measured in the Fourier domain. In order to avoid atmospheric blurring of the fringes, we require a J-band detector capable of fast, low-noise readout, which mandates the use of a SAPHIRA e-APD array. We built a DFS prototype that we tested on-sky at the Magellan Clay telescope behind the MagAO adaptive optics system in May 2018.
The Giant Magellan Telescope (GMT) wavefront control system will provide active optics control and optimized atmospheric turbulence correction to every instrument on the 25.4 m diameter GMT. All subsystems of the GMT wavefront control system have passed their preliminary design reviews, and several are now in the detailed design and prototyping phases. Significant progress has been made developing optimized control algorithms, and simulating observatory performance over a wide range of environmental conditions. We review the wavefront control strategy in each observing mode, and the associated image quality error budgets. We also describe recent d and prototyping progress, and our plans to complete the wavefront control system development and prepare for first light.
The 25.4m Giant Magellan Telescope (GMT) consists of seven 8.4 m primary mirror (M1) segments with matching segmentation of the Gregorian secondary mirror (M2). When operating the GMT in the diffraction-limited Adaptive Optics (AO) modes, using the Adaptive Secondary Mirror (ASM), the M1-M2 pairs of segments must be phased to a small fraction of the observing wavelength. To achieve this level of correction across the scientific field of view (<90” in diameter), the phasing system relies on multiple (up to four) natural guide-star probes deployed across the field of view (from 6’ to 10’ from the center of the field) measuring at slow rates (~0.033 Hz) segment phase piston in the infrared and low-order field-dependent phase aberrations in the visible. This paper describes the overall phasing strategy and requirements when operating in the Natural Guide-star AO (NGAO) and the Laser Tomography AO (LTAO) modes. We will also present a first evaluation of segment piston error induced by wind buffeting on the telescope structure. Wind loads have been computed for different observatory configurations using Computational Fluid Dynamics (CFD) simulations. This analysis showcases the GMT Dynamic Optical Simulation (DOS) environment which integrates the optical and structural dynamic models of the GMT with the Fourier optics models of AO and phasing sensors.
The Giant Magellan Telescope’s Acquisition, Guiding, and Wavefront Sensing System (AGWS) is comprised of four identical probes, each containing 11 axes of precision control. The largest of the mechanisms carries a mass of nearly 500kg. The mechanisms are diverse in type, including a voice coil actuated tip-tilt mirror, a rotary harmonic drive, high accuracy and precision lenslet rotation stages and ballscrew driven linear stages. To meet image quality, positioning, and tracking requirements, these mechanisms and their EtherCATcontrolled servos are designed for stiffness. Employing inductive tape encoders, they must position and track to 10um precision with minimal backlash, over velocities ranging from ~10mm/sec to essentially zero, where stiction becomes significant. We will present the designs of the mechanisms, highlighting key features, design trades, and preliminary prototyping results.
The GMT active optics (AcO) control problem is unique because the primary and secondary mirrors are both segmented. This paper describes the AcO control algorithms and assesses their performance for the Natural Seeing observing modes of the GMT. In this case, there are wavefront sensors in four off-axis probes, which are used to control the position and rotation of each of the seven primary and seven secondary mirror segments, as well as the bending modes of the primary mirror. The segmentation of both mirrors leads to a number of unsensed modes (e.g., segment piston and a rotation of the segment around the telescope optical axis) and poorly sensed modes (e.g., M1 segment translations compensated with the corresponding M2 segment translation). Uncertainties in the location of the probes and guide star coordinates also lead to blind modes. In this paper, we first introduce the GMT optical design in the context of the AcO system. The modes of the telescope controlled by the AcO system, and their counterparts in the wavefront sensing space, are presented next, including field-dependent and blind modes. The control architecture is then outlined along with a description of the singular modes of the system. Finally, performance results are provided with respect to various error terms.
The Acquisition Guiding and Wavefront Sensing System (AGWS) is responsible for making the measurements required to keep the optics of the seven-segment GMT coaligned, phased, pointing in the correct direction, and conforming to the correct mirror shape. The AGWS consists of four identical probes that patrol the outer parts of the GMT field of view. Each probe is comprised of two channels. The visible channel contains optics that can provide high-speed full aperture guiding, segment guiding, or Shack-Hartmann wavefront sensing feeding an EMCCD camera. In natural seeing operations, these probes feed the GMT active optics system. In ground layer AO mode, they are the primary wavefront sensors. The second channel, used for phasing the seven segments in diffraction limited operation, contains J-band dispersed fringe sensor optics feeding a SAPHIRA IR e-APD array. We present the system architecture, and an overview of requirements, optical, mechanical and electrical designs.
The GMT is an aplanatic Gregorian telescope consisting of 7 primary and secondary mirror segments that must be phased to within a fraction of an imaging wavelength to allow the 25.4 meter telescope to reach its diffraction limit. When operating in Laser Tomographic Adaptive Optics (LTAO) mode, on-axis guide stars will not be available for segment phasing. In this mode, the GMT’s Acquisition, Guiding, and Wavefront Sensing system (AGWS) will deploy four pickoff probes to acquire natural guide stars in a 6-10 arcmin annular FOV for guiding, active optics, and segment phasing. The phasing sensor will be able to measure piston phase differences between the seven primary/secondary pairs of up to 50 microns with an accuracy of 50 nm using a J-band dispersed fringe sensor. To test the dispersed fringe sensor design and validate the performance models, SAO has built and commissioned a prototype phasing sensor on the Magellan Clay 6.5 meter telescope. This prototype uses an aperture mask to overlay 6 GMT-sized segment gap patterns on the Magellan 6.5 meter primary mirror reimaged pupil. The six diffraction patterns created by these subaperture pairs are then imaged with a lenslet array and dispersed with a grism. An on-board phase shifter has the ability to simulate an arbitrary phase shift within subaperture pairs. The prototype operates both on-axis and 6 arcmin off-axis either with AO correction from the Magellan adaptive secondary MagAO system on or off in order to replicate as closely as possible the conditions expected at the GMT.
Residual charge generation, or image persistence, in infrared detectors is a problem that affects many low-light astronomical instruments. The HAWAII-2RG in the MMT and Magellan Infrared Spectrograph shows significant persistence when first powered up. We describe here how we reduce the persistence sensitivity of this detector by exposure to light.
The 25.4m Giant Magellan Telescope consists of seven 8.4 m primary mirror (M1) segments with matching segmentation of the Gregorian secondary mirror (M2). When operating the telescope in the diffraction-limited Adaptive Optics (AO) observing modes, the M1-M2 pairs of segments must be phased to a small fraction of the observing wavelength. To achieve this level of correction, the phasing system uses multiple natural guidestar phasing sensors deployed across the field of view to provide an absolute phasing references to edge sensors bridging the gaps between segments. We will present in this paper the performance characterization of the GMT phasing system based on end-to-end numerical simulations performed with the Dynamic Optical Simulation (DOS) tool, which integrates the optical and mechanical dynamics models of the GMT with the Fourier optics models of AO and phasing sensors. The expected phasing performance under different observing conditions will be presented.
The Giant Magellan Telescope (GMT) has a Gregorian 25.4-meter diameter primary mirror composed of seven 8.4-meter diameter segments. The secondary mirror consists of seven 1.1-meter diameter segments. In the active and adaptive operation modes of the GMT, around a dozen wavefront sensors are selectively used to monitor the optical aberrations across the focal plane. A dedicated wavefront control system drives slow and fast corrections at the M1 and M2 mirrors to deliver image quality optimized for the field of view of the scientific instrument in use. This paper describes the control strategies for the active optics mode of the GMT. Different wavefront estimation algorithm are compared and the performance of the GMT is evaluated using the Dynamic Optical Simulation package.
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 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.
The Giant Magellan Telescope active optics system is required to maintain image quality across a 20 arcminute diameter field of view. To do so, it must control the positions of the primary mirror and secondary mirror segments, and the figures of the primary mirror segments. When operating with its adaptive secondary mirror, the figure of the secondary is also controlled. Wavefront and fast-guiding measurements are made using a set of four probes deployed around the field of view. Through a set of simulations we have determined a set of modes that will be used to control fielddependent aberrations without degeneracies.
The conceptual design of the Giant Magellan Telescope has four wavefront sensors used to maintain the shape and alignment of the segmented primary and secondary mirrors. In this paper, we show that by reading the sensors at 200 Hz, we can also compensate for low altitude turbulence. As a result, there is a large improvement in image quality, even at visible wavelengths, over the entire science field of view of the telescope. A minimum-variance reconstructor is presented that takes slope measurements from four stars of arbitrary location and magnitude and produces the optimal adaptive secondary mirror commands. The performance of the adaptive optics system in this mode is simulated using YAO, an end-to-end simulation tool. We present the results of trade studies performed to optimize the science return of the telescope.
The Giant Magellan Telescope (GMT) adaptive optics (AO) system will be an integral part of the telescope, providing laser guidestar generation, wavefront sensing, and wavefront correction to every instrument currently planned on the 25.4 m diameter GMT. There will be three first generation AO observing modes: Natural Guidestar, Laser Tomography, and Ground Layer AO. All three will use a segmented adaptive secondary mirror to deliver a corrected beam directly to the instruments. The Natural Guidestar mode will provide extreme AO performance, with a total wavefront error less than 185 nm RMS using bright guidestars. The Laser Tomography mode uses 6 lasers and a single off-axis natural guidestar to deliver better than 290 nm RMS wavefront error at the science target, over 50% of the sky at the galactic pole. The Ground Layer mode uses 4 natural guidestars on the periphery of the science field to tomographically reconstruct and correct the ground layer AO turbulence, improving the image quality for wide-field instruments. A phasing system maintains the relative alignment of the primary and secondary segments using edge sensors and continuous feedback from an off-axis guidestar. We describe the AO system preliminary design, predicted performance, and the remaining technical challenges as we move towards the start of construction.
NIRMOS (Near-Infrared Multiple Object Spectrograph) is a 0.9 to 2.5 μm imager/spectrograph concept proposed for the
Giant Magellan Telescope<sup>1</sup> (GMT). Near-infrared observations will play a central role in the ELT era, allowing us to
trace the birth and evolution of galaxies through the era of peak star formation. NIRMOS' large field of view, 6.5′ by
6.5′, will be unique among imaging spectrographs developed for ELTs. NIRMOS will operate in Las Campanas' superb
natural seeing and is also designed to take advantage of GMT's ground-layer adaptive optics system. We describe
NIRMOS' high-performance optical and mechanical design.
The f/5 instrumentation suite for the Clay telescope was developed to provide the Magellan Consortium observer community with wide field optical imaging and multislit NIR spectroscopy capability. The instrument suite consists of several major subsystems including two focal plane instruments. These instruments are Megacam and MMIRS. Megacam is a panoramic, square format CCD mosaic imager, 0.4° on a side. It is instrumented with a full set of Sloan filters. MMIRS is a multislit NIR spectrograph that operates in Y through K band and has long slit and imaging capability as well. These two instruments can operate both at Magellan and the MMT. Megacam requires a wide field refractive corrector and a Topbox to support shutter and filter selection functions, as well as to perform wavefront sensing for primary mirror figure correction. Both the corrector and Topbox designs were modeled on previous designs for MMT, however features of the Magellan telescope required considerable revision of these designs. In this paper we discuss the optomechanical, electrical, software and structural design of these subsystems, as well as operational considerations that attended delivery of the instrument suite to first light.
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.
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.
The design of the adaptive optics (AO) system for the GMT is currently being developed. The baseline system is
planned around a segmented adaptive secondary mirror (ASM), with elements similar in size to current ASM's for 8 m
telescopes. A facility wavefront sensing system is planned to provide AO correction at several science instrument ports.
The AO system will contain a subsystem dedicated to controlling the relative phases between the seven segments of the
GMT aperture. The anticipated modes include natural guide star, laser tomography, and ground layer adaptive optics. A
cooled optical relay is described to provide baffling and reimaging of the focal plane to the various science ports. The
laser projection system will use six beacons on an adjustable radius to support both diffraction-limited and ground layer
correction modes. Modeling work, as well as science instrument design development will be integrated with this design
effort to develop a concept that provides efficient diffraction-limited performance and seeing-improved capabilities for
Modeling adaptive optics (AO) systems is crucial to understanding their performance and a key aid in their
design. The Giant Magellan Telescope (GMT) is planning three AO modes at first light: natural guide star AO,
ground-layer AO and laser tomography AO. This paper describes how a modified version of YAO, an open-source
general-purpose AO simulation tool written in Yorick, is used to simulate the GMT AO modes. The simulation
tool was used to determine the piston segment error for the GMT. In addition, we present a comparison of
different turbulence simulation approaches.
The Smithsonian Widefield Infrared Camera (SWIRC) is a Y -, J-, and H-band imager for the f/5 MMT.
Proposed in May 2003 and commissioned in June 2004, the goal of the instrument was to deliver quickly a wide
field-of-view instrument with minimal optical elements and hence high throughput. The trade-off; was to sacrifice
K-band capability by not having an internal, cold Lyot stop. We describe SWIRC's design and capabilities, and
discuss lessons learned from the thermal design and the detector mount, all of which have been incorporated into
the upcoming MMT & Magellan Infrared Spectrograph.
The converted 6.5m MMT Observatory has a powerful suite of new instrumentation accumulated over the last eight
years. Pre-conversion instruments still in use at the f/9 Cassegrain focus are the facility Red and Blue Channel
spectrographs (R = 240 - 6600) and the visiting spectropolarimeter (SPOL). Instruments using the f/5 spectroscopic
configuration are the bench mounted 300-fiber spectrographs Hectospec (R=1000) and Hectochelle (R=30,000), and the
single slit, cross-dispersed spectrograph MAESTRO (R=28,000 - 93,000). The f/5 imaging configuration offers
Megacam, a 24' x 24' CCD mosaic camera and SWIRC, a YJH NIR imager. The MMT's pioneering f/15 adaptive
secondary mirror enables high-resolution imaging and spectroscopy in the infrared with the ARIES, CLIO, PISCES and
BLINC/MIRAC instruments. The AO system will shortly be significantly enhanced with the addition of a Rayleigh laser
guide star system which is currently being commissioned. Upcoming instrumentation will include slit mask
spectrographs in the infrared (MMIRS) and optical (BINOSPEC). This review paper presents all the available
instruments capabilities and demonstrates how the observatory has become highly efficient at managing multiple
secondary mirrors and a large instrument suite.
The Giant Magellan Telescope, with seven 8.4 meter primary mirrors, is taking shape as one of the most powerful
telescopes of the next generation. We describe a conceptual design for a powerful 0.85 to 2.50 μm imaging
spectrograph that addresses a 7' by 7' field of view for imaging and a 5' by 7' field of view for spectroscopy at the
GMT's f/8 Gregorian focus. The all-refractive optical design presses the limits of available lens blank diameters, but
delivers excellent images (~0.15" 80% encircled energy) with just four collimator elements and five camera elements.
The collimated beam diameter is 300 mm, and the detector is a 6K by 10K array. The spectrograph will use
interchangeable slit masks, and an assortment of VPH and conventional surface relief gratings. Each of the entire J, H,
or K bands can be observed with a resolution of 3000. The scientific potential of ground layer adaptive optics (GLAO)
using a constellation of sodium laser guide stars appears to be very high in the near infrared. Simulations suggest that
0.2" FWHM images may be achieved across the entire 7' by 7' field of view of the spectrograph. We describe the
design of the GLAO system with a versatile opto-mechanical design that allows rapid changeover between GLAO and
In 2003, the converted MMT’s wide-field f/5 focus was commissioned. A 1.7-m diameter secondary and a large refractive corrector offer a 1° diameter field of view for spectroscopy and a 0.5° diameter field of view for imaging. Stellar images during excellent seeing are smaller than 0.5" FWHM across the spectroscopic field of view, and smaller than 0.4" across the imaging field of view. Three wide-field f/5 instruments are now in routine operation: Hectospec (an R~1000 optical spectrograph fed by 300 robotically-positioned optical fibers), Hectochelle (an R~40,000 optical spectrograph fed by the same fibers), and Megacam (a 340 megapixel, 36 CCD optical imager covering a 25' by 25' format).
We present the preliminary design for the MMT and Magellan Infrared
Spectrograph (MMIRS). MMIRS is a fully refractive imager and multi-object spectrograph that uses a 2048x2048 pixel Hawaii2 HgCdTe array. It offers a 7'x7' imaging field of view and a 4'x7' field of view for multi-object spectroscopy. Dispersion is provided by a set of 5 grisms providing R=3000 at J, H, or K, or R=1300 in J+H or H+K.
We report on new results of simultaneous measurements of sodium layer column density and the absolute return flux from laser guide stars created by a monochromatic approximately 1 W CW laser, tuned to the peak of the sodium D2 hyperfine structure. The return was measured at the MMT while the sodium abundance was measured at the CFA 60 inch telescope, about 1 km away, with the Advanced Fiber Optic Echelle spectrograph. The laser frequency stability, which can greatly affect the return flux, was monitored at the same time in order to improve the measurement accuracy. After the correction for laser frequency jitter and atmospheric transmission, the absolute flux return above the atmosphere for circularly polarized light is 1.2 X 10<SUP>6</SUP> photons s<SUP>-1</SUP> m<SUP>-2</SUP> per watt launched above the atmosphere, per unit column density, which we taken as our measured mean over the year of N(Na) equals 3.7 X 10<SUP>9</SUP> cm<SUP>-2</SUP> at Tucson. The solidification of a final well-determined relationship between the sodium laser guide star brightness and sodium layer column density is pivotal in the design of the next generation laser guide star adaptive optics systems. We also report the measurements and analysis of the relationship between the projected beam waist of the sodium laser and the resultant spot size on the sodium layer under typical atmospheric conditions. Since wavefront measured error is proportional to spot size, and also to 1/(root) power, minimum spot size is crucial for lowest laser power requirement. By projecting the laser through diffraction limited optics of 0.5 m diameter, roughly 3 r<SUB>0</SUB>, we have achieved the smallest artificial beacon yet recorded, about 0.8 arcsec.
The Hectospec is a moderate dispersion spectrograph fed by 300 optical fibers. Hectospec's pair of five-axis robots will position fibers at the 1 degree diameter f/5 focus of the converted MMT, allowing efficient multi-object spectroscopy. We discuss algorithms that we have developed to match the optical fibers to celestial objects and then to compute the appropriate sequence of robotic positioner moves to reconfigure the fibers between successive observations. Both algorithms require essentially no user interaction, consume only modest computer resources and allow effective deployment of the Hectospec's 300 fibers. The target-to-fiber matching algorithm is a recursive procedure which allows simultaneous optimization of the multiple observations that are required to complete a large survey. The robotic motion sequence algorithm allows the two Hectospec robots to work together efficiently to move fibers directly between observing configurations.
Megacam is a 36 CCD mosaic camera that will cover a 24' X 24' field of view at the f/5 wide-field focus of the converted 6.5 m Multiple Mirror Telescope. The mosaic is a 9 X 4 array of thinned 2048 X 4068 pixel CCDs with 13.5 micrometer pixels. The CCDs are dual-output EEV devices in a custom package to allow the devices to be closely butted on all four sides. The dewar will be mounted to a 2 m diameter assembly that contains the filter wheels (for 30 X 30 cm filters) and the shutter. Telescope guiding will be accomplished with two additional CCDs mounted at the edges of the focal plane. The guider CCDs will be operated slightly defocused, one on either side of focus, to allow simultaneous focusing and guiding. Guide stars will be selected by reading out the full guider frame, after which only a small area surrounding the guide star will be read out. Our simulations show that the defocused guide star images will also be useful for low order wavefront sensing, allowing corrections to the telescope collimation. We are developing a new CCD controller capable of reading the full Megacam in 24 seconds. This controller will also be used to operate the guide chips.
Megacam is a wide-field optical imager for the converted MMT that uses thirty-six 2048 by 4608 pixel CCDs to cover a 24' X 24' format. We describe a computer architecture designed to accommodate the expected data volume and show benchmark results from prototype implementations that demonstrate the performance attained by each of the design decisions. We show that our time budget allotments can be met using a modular, scalable architecture design that exploits the natural parallelization of multiple, identical detector components.
The FASTTRAC II adaptive optics instrument has been used at the Multiple Mirror Telescope (MMT) for the past 2 years to provide improved image resolution in the near infrared. Results have been obtained using both natural guide stars and an artificial sodium laser beacon. With the imminent closure of the MMT prior to its conversion to a single-mirror 6.5 m telescope, FASTTRAC II has come to the end of its life. The instrument has been to the telescope for a total of 8 runs, and during that time it has been of enormous value both as a learning aid, demonstrating the requirements of its successor on the 6.5 m, and as a scientific tool. At this meeting, we present a selection of astrophysical data derived from FASTTRAC II, including the first closed-loop demonstration of an adaptive optics system using a sodium laser beacon. The sodium laser has been used to obtain near diffraction-limited near-infrared images of the core of M13, allowing the construction of a color-magnitude diagram to below the main sequence turnoff. Results have also been obtained from several gravitationally lensed quasars, and the cores of nearby galaxies in the local group. We also summarize work characterizing atmospheric conditions at the site. These studies have proceeded in two areas - understanding the behavior of the phase perturbation with field angle and time, and characterizing the return from the sodium resonance beacon.
The conversion of the Multiple Mirror Telescope from six 1.8 m primary mirrors to a single 6.5 m primary will significantly increase its capability for imaging. The f/5 configuration will provide a corrected field of view for imaging that is flat and 30 arcminutes in diameter. The image quality in the absence of atmospheric seeing is 0'.1 over the full field. We are currently designing a camera system to take advantage of this large field. The proposed direct imaging system will be located at the Cassegrain focus of the telescope, behind a three-element refractive corrector. We will use an array of 8 X 4 three-edge-buttable CCDs, each with 2048 X 4096 pixels and two output amplifiers. This will provide a field of view of 24' X 24'. With a new packaging scheme we will reduce the gap along the readout edge to a few millimeters. The pixel size is 15 microns, or 0'.09, well sampling the point-spread- function. In many applications it will be possible to bin the pixels, thus reducing the amount of data (500 Mb per read at full resolution). The back-illuminated CCDs will be thinned and anti- reflection coated to provide high quantum efficiency from 320 to 1000 nm. The camera system will be useful for many studies requiring both a large collecting area and large area coverage on the sky. Planned projects include redshift and photometric surveys of faint galaxies, searches for high-redshift quasars and searches for objects in the outer solar system.
A new adaptive optics system has been constructed for moderately high resolution in the near infrared at the Multiple Mirror Telescope (MMT). The system, called FASTTRAC II, has been designed to combine the highest throughput with the lowest possible background emission by making the adaptive optical element be an existing and necessary part of the telescope, and by eliminating all warm surfaces between the telescope and the science camera's dewar. At present, only natural guide stars are supported, but by the end of 1995, we will add the capability to use a single sodium resonance beacon derived from a laser beam projected nearly coaxially with the telescope. In this paper, we present a description of FASTTRAC II, and show results from its first test run at the telescope in April 1995.
Low spatial frequencies of atmospheric turbulence are specially troublesome to astronomers because the phase distortions they cause have large amplitude. We have begun experiments at the Multiple Mirror Telescope (MMT) to remove these errors with tip, tilt, and piston control of pieces of the wave front defined by the telescope's six 1.8 m primary mirrors. We show long exposure images taken at the telescope with resolution as high as 0.08 arcsec under piston control, and 0.26 arcsec under tilt control, using an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. We also present preliminary results from analysis of images of the pre-main sequence star T Tauri taken with tilt control of the six beams only, at three infrared wavelengths. The resolution is between 0.35 and 0.4 arcsec, higher than has previously been achieved with direct imaging. The faint red companion to T Tau is clearly revealed, and is seen to be undergoing an energetic outburst.
The next generation of 6 to 10 m class telescopes is being planned to include the capability for adaptive wavefront correction. The MMT with its 7-m baseline, provides an ideal testbed for novel techniques of adaptive optics. Using a new instrument based on a six-segment adaptive mirror, a number of wavefront sensing algorithms including an artificial neural network have been implemented to demonstrate the high resolution imaging capability of the telescope. These algorithms rely on a freely available property of starlight, namely, its coherence over large scales, to sense directly atmospheric and instrumental phase errors across large distances. In this paper, we report results obtained so far with resolutions between 0.08 and 0.3 arcsec at 2.2-micron wavelength. We also show data indicating that at the level of 0.1-arcsec imaging in good seeing, the isoplanatic patch at this wavelength is at least 20 arcsec across.
The MMT consists of six comounted 1.8 m telescopes from which the light is brought to a combined coherent focus. Atmospheric turbulence spoils the MMT diffraction-limited beam profile, which would otherwise have a central peak of 0.06 arcsec FWHM, at 2 microns wavelength. At this wavelength, the adaptive correction of the tilt and path difference of each telescope beam is sufficient to recover diffraction-limited angular resolution. Computer simulations have shown that these tilts and pistons can be derived by an artificial neural network, given only a simultaneous pair of in-focus and out-of-focus images of a reference star formed at the combined focus of all the array elements. We describe such an adaptive optics system for the MMT, as well as some successful tests of neural network wavefront sensing on images, and initial real-time tests of the adaptive system at the telescope; attention is given to a demonstration of the adaptive stabilization of the mean phase errors between two mirrors which resulted in stable fringes with 0.1 arcsec resolution.
An infrared camera has been developed for interferometric imaging on large telescopes. Observations obtained with the 6.86 m aperture of the cophased Multiple Mirror Telescope (MMT) demonstrate the ease with which future 8 m telescopes can achieve diffraction-limited performance from 1 to 5.5 micron. With the MMT, the infrared camera has imaged astronomical sources at 3.5 micron with a diffraction-limited resolution of 0.10 arcsec. Adjustable exposures as short as 4 msec are obtained at a maximum rate of 10 Hz to freeze atmospheric turbulence.