We compare the optical performance, alignment sensitivity, and thermal stability of a Non-Uniform Rational B-Spline (NURBS) freeform telescope design to two more conventional design forms with the goal of facilitating acceptance of this new optical surface for aerospace applications. We present the designs of three three-mirror anastigmat (TMA) wide field (4°) telescopes with identical first order optical design parameters. These TMAs consist of a conventional design using off-axis aspheric mirrors, a freeform design using off-axis Zernike polynomial surfaces, and a freeform design using NURBS surfaces. Of the three, the NURBS design gives the best image quality and lowest geometrical design residual. The three designs have similar misalignment sensitivities and sensitivity to thermal soaks, countering a common misconception that freeform designs are more sensitive to misalignment than conventional designs.
The Wide Field Phasing Testbed for the Giant Magellan Telescope1 is comprised of six pairs of off-axis parabolas, an Offner relay and several additional fold mirrors. To align this optical system we used a Leica laser tracker and a 4D interferometer. The laser tracker was used to accurately position each fold mirror on the optical bench by measuring the position of a spherically mounted retroreflector with the laser tracker, first directly and then in reflection off the mirror to be aligned. The mirror was adjusted until the reflected image was in the correct location. Key to this operation was custom software that read in the Zemax prescription file specifying the location of each optic; interfaced with the laser tracker to measure the location of fiducial SMRs on the optical bench; performed transformations between coordinate systems attached to the laser tracker, the optical bench, the Zemax model, and the individual optics; and finally displayed the real-time position errors in a large font so that the optic could be easily adjusted to the correct location. The OAPs were also positioned using the laser tracker in conjunction with the interferometer. An SMR was placed at the desired focal position of the OAP using the laser tracker. This same SMR served as the return sphere for the interferometer which was used to adjust out tilt and astigmatism errors. With this system we were able to align the full optical system efficiently and in a deterministic way.
The Wide Field Phasing Testbed will be used to test phasing and active optics systems planned for the doubly segmented Giant Magellan Telescope. The testbed consists of a set of optical relays in which are located segmented and deformable mirrors that represent the GMT M1 and M2 mirrors. The testbed output beam has the GMT’s f/8.16 focal ratio and has a back focal distance large enough to allow using a full-scale prototype of one unit of the Acquisition Guiding and Wavefront Sensing System. The testbed will reproduce the telescope field dependent aberrations that result from misalignment of M1 and M2. Over its 20mm diameter field of view, the testbed will generate aberrations corresponding to the 20′ field of the GMT. A rotating turbulence screen and zero-deviation prisms in the testbed will generate seeing limited images that correspond to typical atmospheric seeing and dispersion conditions expected at the GMT. The software for the testbed is designed to allow connection of the testbed wavefront sensing analysis components to simulations of the testbed optical system, as well as to conform to the planned software interfaces of the GMT’s telescope control system.
The GMT Acquisition, Guidance and Wavefront Sensing System (AGWS) is responsible for making the measurements
required to keep the optics of the seven-segment Giant Magellan Telescope coaligned, phased, pointed correctly and
properly figured. Each AGWS probe includes several mechanisms to enable the probe to access and accurately track guide
stars within its patrol field. Mechanism performance is crucial to the overall performance of the AGWS as they must be
able to handle the large mass of the probe while operating over the wide GMT operational temperature and dynamic motion
ranges. Prototyping test results have demonstrated compliance with challenging AGWS requirements.
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 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 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 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.
We are developing a stable and precise spectrograph for the Large Binocular Telescope (LBT) named “iLocater.” The instrument comprises three principal components: a cross-dispersed echelle spectrograph that operates in the YJ-bands (0.97-1.30 μm), a fiber-injection acquisition camera system, and a wavelength calibration unit. iLocater will deliver high spectral resolution (R~150,000-240,000) measurements that permit novel studies of stellar and substellar objects in the solar neighborhood including extrasolar planets. Unlike previous planet-finding instruments, which are seeing-limited, iLocater operates at the diffraction limit and uses single mode fibers to eliminate the effects of modal noise entirely. By receiving starlight from two 8.4m diameter telescopes that each use “extreme” adaptive optics (AO), iLocater shows promise to overcome the limitations that prevent existing instruments from generating sub-meter-per-second radial velocity (RV) precision. Although optimized for the characterization of low-mass planets using the Doppler technique, iLocater will also advance areas of research that involve crowded fields, line-blanketing, and weak absorption lines.
We review astronomical results in the visible (λ<1μm) with adaptive optics. Other than a brief period in the early 1990s, there has been little astronomical science done in the visible with AO until recently. The most productive visible AO system to date is our 6.5m Magellan telescope AO system (MagAO). MagAO is an advanced Adaptive Secondary system at the Magellan 6.5m in Chile. This secondary has 585 actuators with < 1 msec response times (0.7 ms typically). We use a pyramid wavefront sensor. The relatively small actuator pitch (~23 cm/subap) allows moderate Strehls to be obtained in the visible (0.63-1.05 microns). We use a CCD AO science camera called “VisAO”. On-sky long exposures (60s) achieve <30mas resolutions, 30% Strehls at 0.62 microns (r') with the VisAO camera in 0.5” seeing with bright R < 8 mag stars. These relatively high visible wavelength Strehls are made possible by our powerful combination of a next generation ASM and a Pyramid WFS with 378 controlled modes and 1000 Hz loop frequency. We'll review the key steps to having good performance in the visible and review the exciting new AO visible science opportunities and refereed publications in both broad-band (r,i,z,Y) and at Halpha for exoplanets, protoplanetary disks, young stars, and emission line jets. These examples highlight the power of visible AO to probe circumstellar regions/spatial resolutions that would otherwise require much larger diameter telescopes with classical infrared AO cameras.
We present descriptions of the alignment and calibration tests of the Pathfinder, which achieved first light during our 2013 commissioning campaign at the LBT. The full LINC-NIRVANA instrument is a Fizeau interferometric imager with fringe tracking and 2-layer natural guide star multi-conjugate adaptive optics (MCAO) systems on each eye of the LBT. The MCAO correction for each side is achieved using a ground layer wavefront sensor that drives the LBT adaptive secondary mirror and a mid-high layer wavefront sensor that drives a Xinetics 349 actuator DM conjugated to an altitude of 7.1 km. When the LINC-NIRVANA MCAO system is commissioned, it will be one of only two such systems on an 8-meter telescope and the only such system in the northern hemisphere. In order to mitigate risk, we take a modular approach to commissioning by decoupling and testing the LINC-NIRVANA subsystems individually. The Pathfinder is the ground-layer wavefront sensor for the DX eye of the LBT. It uses 12 pyramid wavefront sensors to optically co-add light from natural guide stars in order to make four pupil images that sense ground layer turbulence. Pathfinder is now the first LINC-NIRVANA subsystem to be fully integrated with the telescope and commissioned on sky. Our 2013 commissioning campaign consisted of 7 runs at the LBT with the tasks of assembly, integration and communication with the LBT telescope control system, alignment to the telescope optical axis, off-sky closed loop AO calibration, and finally closed loop on-sky AO. We present the programmatics of this campaign, along with the novel designs of our alignment scheme and our off-sky calibration test, which lead to the Pathfinder’s first on-sky closed loop images.
The LINC-NIRVANA Pathfinder1 (LN-PF), a ground-layer adaptive optics (AO) system recently commissioned at the Large Binocular Telescope (LBT), is one of 4 sensors that provide AO corrected images to the full LINC-NIRVANA instrument. With first light having taken place on November 17, 2013,2, 3 the core goals for the LN-PF have been accomplished. In this report, we look forward to one of the LN-PF extended goals. In particular, we review the acquisition mechanism required to place each of several star probes on its corresponding star in the target asterism. For emerging AO systems in general, co-addition of light from multiple stars stands as one of several methods being pursued to boost sky coverage. With 12 probes patrolling a large field of view (an annulus 6-arcminutes in diameter), the LN-PF will provide a valuable testbed to verify this method.
MagAO is the new adaptive optics system with visible-light and infrared science cameras, located on the 6.5-m Magellan “Clay” telescope at Las Campanas Observatory, Chile. The instrument locks on natural guide stars (NGS) from 0th to 16th R-band magnitude, measures turbulence with a modulating pyramid wavefront sensor binnable from 28×28 to 7×7 subapertures, and uses a 585-actuator adaptive secondary mirror (ASM) to provide at wavefronts to the two science cameras. MagAO is a mutated clone of the similar AO systems at the Large Binocular Telescope (LBT) at Mt. Graham, Arizona. The high-level AO loop controls up to 378 modes and operates at frame rates up to 1000 Hz. The instrument has two science cameras: VisAO operating from 0.5-1μm and Clio2 operating from 1-5 μm. MagAO was installed in 2012 and successfully completed two commissioning runs in 2012-2013. In April 2014 we had our first science run that was open to the general Magellan community. Observers from Arizona, Carnegie, Australia, Harvard, MIT, Michigan, and Chile took observations in collaboration with the MagAO instrument team. Here we describe the MagAO instrument, describe our on-sky performance, and report our status as of summer 2014.
One of the primary goals of exoplanet science is to find and characterize habitable planets, and direct imaging will play a key role in this effort. Though imaging a true Earth analog is likely out of reach from the ground, the coming generation of giant telescopes will find and characterize many planets in and near the habitable zones (HZs) of nearby stars. Radial velocity and transit searches indicate that such planets are common, but imaging them will require achieving extreme contrasts at very small angular separations, posing many challenges for adaptive optics (AO) system design. Giant planets in the HZ may even be within reach with the latest generation of high-contrast imagers for a handful of very nearby stars. Here we will review the definition of the HZ, and the characteristics of detectable planets there. We then review some of the ways that direct imaging in the HZ will be different from the typical exoplanet imaging survey today. Finally, we present preliminary results from our observations of the HZ of α Centauri A with the Magellan AO system’s VisAO and Clio2 cameras.
The LINC-NIRVANA Pathfinder experiment is a test-bed to verify a very complex sub-system: the Ground-layer Wavefront Sensor, or GWS. Pathfinder will test the GWS in its final working environment and demonstrate on-sky the performance achievable with a multiple natural guide star, ground-layer adaptive optics system with a very wide FoV. The GWS uses up to 12 natural guide stars within a 2.8'-6' annular field of view and drives the LBT adaptive secondary mirror to correct the lower layers of atmospheric turbulence. This paper will trace the path of the instrument on its way to First Light on-sky in November 2013, from its installation on the telescope to the calibrations to its final operation, focusing in particular on opto-mechanical and software aspects and how they lead to the main achieved results.
The Magellan AO system combines a pyramid wavefront sensor and high-order adaptive secondary mirror, and
will see first light on the Magellan Clay telescope in November 2012. With a 24 cm projected actuator pitch,
this powerful system will enable good correction in the optical (0.5 to 1 μm). Realistic laboratory testing has
produced Strehl ratios greater than 40% in i’ (0.765 μm) on bright simulated stars. On fainter stars our visible
AO camera, VisAO, will work in the partially corrected regime with only short moments of good correction.
We have developed a form of lucky imaging, called real time frame selection, which uses a fast shutter to block
moments of bad correction, and quickly opens the shutter when the correction is good, enabling long integrations
on a conventional CCD while maximizing Strehl ratio and resolution. The decision to open or shut is currently
based on reconstructed WFS telemetry. Here we report on our implementation and testing of this technique in
the Arcetri test tower in Florence, Italy, where we showed that long exposure i’ Strehl could be improved from
16% to 26% when the selection threshold was set to the best 10% of instantaneous Strehl.
The heart of the 6.5 Magellan AO system (MagAO) is a 585 actuator adaptive secondary
mirror (ASM) with <1 msec response times (0.7 ms typically). This adaptive secondary will
allow low emissivity and high-contrast AO science. We fabricated a high order (561 mode)
pyramid wavefront sensor (similar to that now successfully used at the Large Binocular
Telescope). The relatively high actuator count (and small projected ~23 cm pitch) allows
moderate Strehls to be obtained by MagAO in the “visible” (0.63-1.05 μm). To take advantage
of this we have fabricated an AO CCD science camera called "VisAO". Complete “end-to-end”
closed-loop lab tests of MagAO achieve a solid, broad-band, 37% Strehl (122 nm rms) at 0.76
μm (i’) with the VisAO camera in 0.8” simulated seeing (13 cm ro at V) with fast 33 mph
winds and a 40 m Lo locked on R=8 mag artificial star. These relatively high visible
wavelength Strehls are enabled by our powerful combination of a next generation ASM and a
Pyramid WFS with 400 controlled modes and 1000 Hz sample speeds (similar to that used
successfully on-sky at the LBT). Currently only the VisAO science camera is used for lab
testing of MagAO, but this high level of measured performance (122 nm rms) promises even
higher Strehls with our IR science cameras. On bright (R=8 mag) stars we should achieve very
high Strehls (>70% at H) in the IR with the existing MagAO Clio2 (λ=1-5.3 μm) science
camera/coronagraph or even higher (~98% Strehl) the Mid-IR (8-26 microns) with the existing
BLINC/MIRAC4 science camera in the future. To eliminate non-common path vibrations,
dispersions, and optical errors the VisAO science camera is fed by a common path advanced
triplet ADC and is piggy-backed on the Pyramid WFS optical board itself. Also a high-speed
shutter can be used to block periods of poor correction. The entire system passed CDR in June
2009, and we finished the closed-loop system level testing phase in December 2011. Final
system acceptance (“pre-ship” review) was passed in February 2012. In May 2012 the entire
AO system is was successfully shipped to Chile and fully tested/aligned. It is now in storage in
the Magellan telescope clean room in anticipation of “First Light” scheduled for December
2012. An overview of the design, attributes, performance, and schedule for the Magellan AO
system and its two science cameras are briefly presented here.
We present laboratory results of the closed-loop performance of the Magellan Adaptive Optics (AO) Adaptive
Secondary Mirror (ASM), pyramid wavefront sensor (PWFS), and VisAO visible adaptive optics camera. The Magellan
AO system is a 585-actuator low-emissivity high-throughput system scheduled for first light on the 6.5 meter Magellan
Clay telescope in November 2012. Using a dichroic beamsplitter near the telescope focal plane, the AO system will be
able to simultaneously perform visible (500-1000 nm) AO science with our VisAO camera and either 10 μm or 3-5 μm
science using either the BLINC/MIRAC4 or CLIO cameras, respectively. The ASM, PWS, and VisAO camera have
undergone final system tests in the solar test tower at the Arcetri Institute in Florence, Italy, reaching Strehls of 37% in
i'-band with 400 modes and simulated turbulence of 14 cm ro at v-band. We present images and test results of the assembled VisAO system, which includes our prototype advanced Atmospheric Dispersion Corrector (ADC), prototype
calcite Wollaston prisms for SDI imaging, and a suite of beamsplitters, filters, and other optics. Our advanced ADC
performs in the lab as designed and is a 58% improvement over conventional ADC designs. We also present images and
results of our unique Calibration Return Optic (CRO) test system and the ASM, which has successfully run in closedloop
at 1kHz. The CRO test is a retro reflecting optical test that allows us to test the ASM off-sky in close-loop using an
artificial star formed by a fiber source.
We present the optomechanical design of the Magellan VisAO Integral Field Spectrograph (VisAO IFS),
designed to take advantage of Magellan's AO system and its 85.1cm concave ellipsoidal Adaptive Secondary Mirror
(ASM). With 585 actuators and an equal number of actively-controlled modes, this revolutionary second generation
ASM will be the first to achieve moderate Strehl ratios into the visible wavelength regime. We have designed the VisAO
IFS to be coupled to either Magellan's LDSS-3 spectrograph or to the planned facility M2FS fiber spectrograph and to
optimize VisAO science. Designed for narrow field-of-view, high spatial resolution science, this lenslet-coupled fiberfed
IFS will offer exciting opportunities for scientific advancement in a variety of fields, including protoplanetary disk
morphology and chemistry, resolution and spectral classification of tight astrometric binaries, seasonal changes in the
upper atmosphere of Titan, and a better understanding of the black hole M-sigma relation.
The Magellan AO system will begin commissioning in early 2012. Its VisAO camera will provide 20 mas
FWHM images with mean Strehl ratios of ~ 0.2 in R band on a 6.5m telescope. Depending on seeing conditions,
Strehl ratio may reach temporary peaks as high as 0.5 at these wavelengths. To take advantage of these brief
periods of high performance, we plan to adopt lucky imaging style data taking and reduction techniques. As part
of this effort we have developed a novel real-time frame selection technique, which will use AO system telemetry
and a fast shutter to limit CCD exposure to these very brief moments of higher Strehl. Here we describe the
expected benefits of our frame selection techniques in various operating modes. We also present the results of
laboratory characterization of the shutter, and describe the performance of predictive algorithms used to control
it.
The Magellan Adaptive Secondary AO system, scheduled for first light in the fall of 2011, will be able to simultaneously
perform diffraction limited AO science in both the mid-IR, using the BLINC/MIRAC4 10μm camera, and in the visible
using our novel VisAO camera. The VisAO camera will be able to operate as either an imager, using a CCD47 with 8.5
mas pixels, or as an IFS, using a custom fiber array at the focal plane with 20 mas elements in its highest resolution
mode. In imaging mode, the VisAO camera will have a full suite of filters, coronagraphic focal plane occulting spots,
and SDI prism/filters. The imaging mode should provide ~20% mean Strehl diffraction-limited images over the band
0.5-1.0 μm. In IFS mode, the VisAO instrument will provide R~1,800 spectra over the band 0.6-1.05 μm. Our
unprecedented 20 mas spatially resolved visible spectra would be the highest spatial resolution achieved to date, either
from the ground or in space. We also present lab results from our recently fabricated advanced triplet Atmospheric
Dispersion Corrector (ADC) and the design of our novel wide-field acquisition and active optics lens. The advanced
ADC is designed to perform 58% better than conventional doublet ADCs and is one of the enabling technologies that
will allow us to achieve broadband (0.5-1.0μm) diffraction limited imaging and wavefront sensing in the visible.
The Magellan Clay telescope is a 6.5m Gregorian telescope located in Chile at Las Campanas Observatory.
The Gregorian design allows for an adaptive secondary mirror that can be tested off-sky in a straightforward
manner. We have fabricated a 85 cm diameter aspheric adaptive secondary with our subcontractors
and partners, the ASM passed acceptance tests in July 2010. This secondary has 585 actuators with <1
msec response times (0.7 ms typically). This adaptive secondary will allow low emissivity AO science. We
will achieve very high Strehls (~98%) in the Mid-IR (3-26 microns) with the BLINC/MIRAC4 Mid-IR
science camera. This will allow the first "super-resolution" and nulling Mid-IR studies of dusty southern
objects. We will employ a high order (585 mode) pyramid wavefront sensor similar to that now
successfully used at the Large Binocular Telescope. The relatively high actuator count will allow modest
Strehls to be obtained in the visible (0.63-1.05 μm). Moderate (~20%) Strehls have already been obtained
at 0.8 μm at the LBT with the same powerful combination of a next generation ASM and Pyramid WFS as
we are providing for Magellan. Our visible light AO (VisAO) science camera is fed by an advanced triplet
ADC and is piggy-backed on the WFS optical board. We have designed an additional "clean-up" very fast
(2 kHz) tilt stabilization system for VisAO. Also a high-speed shutter will be used to block periods of poor
correction. The VisAO facility can be reconfigured to feed an optical IFU spectrograph with 20 mas
spaxels. The entire system passed CDR in June 2009, and is
now finished the fabrication phase and is entering the
integration phase. The system science and performance
requirements, and an overview the design, interface and
schedule for the Magellan AO system are presented here.
Since its beginnings, diffraction-limited ground-based adaptive optics (AO) imaging has been limited to wavelengths in the near IR (λ>1μm) and longer. Visible AO (λ>1μm) has proven to be difficult because shorter wavelengths require wavefront correction on very short spatial and temporal scales. The pupil must be sampled very finely, which requires dense actuator spacing and fine wavefront sampling with large dynamic range. In addition, atmospheric dispersion is much more significant in the visible than in the near-IR. Imaging over a broad visible band requires a very good Atmospheric Dispersion Corrector (ADC). Even with these technologies, our AO simulations using the CAOS code, combined with the optical and site parameters for the 6.5m Magellan telescope, demonstrate a large temporal variability of visible (λ=0.7μm) Strehl on timescales of 50 ms. Over several hundred milliseconds, the visible Strehl can be as high
at 50% and as low as 10%. Taking advantage of periods of high Strehl requires either the ability to read out the CCD very fast, thereby introducing significant amounts of read-noise, or the use of a fast asynchronous shutter that can block the low-Strehl light. Our Magellan VisAO camera will use an advanced ADC, a high-speed shutter, and our 585 actuator adaptive secondary to achieve broadband (0.5-1.0 μm) diffraction limited images on the 6.5m Magellan Clay telescope in Chile at Las Campanas Observatory. These will be the sharpest and deepest visible direct images taken to
date with a resolution of 17 mas, a factor of 2.7 better than the diffraction limit of the Hubble Space Telescope.
The Magellan Clay telescope is a 6.5m Gregorian telescope located in southern Chile at Las Campanas
Observatory. The Gregorian design allows for an adaptive secondary mirror that can be tested off-sky in a
straight-forward manner. We have fabricated a 85 cm diameter aspheric adaptive secondary with our
subcontractors and partners. This secondary has 585 actuators with <1 msec response times. The chopping
adaptive secondary will allow low emissivity AO science. We will achieve very high Strehls (~98%) in the
Mid-IR AO (8-26 microns) with the BLINC/MIRAC4 Mid-IR science camera. This will allow the first
"super-resolution" and nulling Mid-IR studies of dusty southern objects. We will employ a high order (585
mode) pyramid wavefront sensor similar to that used in the Large Binocular Telescope AO systems. The
relatively high actuator count will allow modest Strehls to be obtained in the visible (~0.8μm). Our visible
light AO (Vis AO) science camera is fed by an advanced ADC and beamsplitter piggy-backed on the WFS
optical table. The system science and performance requirements, and an overview the design, interface and
schedule for the Magellan AO system are presented here.
In addition to the BLINC/MIRAC IR science instruments, the Magellan adaptive secondary AO system will have an
EEV CCD47 that can be used both for visible AO science and as a wide-field acquisition camera. The effects of
atmospheric dispersion on the elongation of the diffraction limited Magellan adaptive optics system point spread
function (PSF) are significant in the near IR. This elongation becomes particularly egregious at visible wavelengths,
culminating in a PSF that is 2000&mgr;m long in one direction and diffraction limited (30-60 &mgr;m) in the other over the
wavelength band 0.5-1.0&mgr;m for a source at 45° zenith angle. The planned Magellan AO system consists of a deformable
secondary mirror with 585 actuators. This number of actuators should be sufficient to nyquist sample the atmospheric
turbulence and correct images to the diffraction limit at wavelengths as short as 0.7&mgr;m, with useful science being
possible as low as 0.5&mgr;m. In order to achieve diffraction limited performance over this broad band, 2000&mgr;m of lateral
color must be corrected to better than 10&mgr;m. The traditional atmospheric dispersion corrector (ADC) consists of two
identical counter-rotating cemented doublet prisms that correct the primary chromatic aberration. We propose two new
ADC designs: the first consisting of two identical counter-rotating prism triplets, and the second consisting of two pairs
of cemented counter-rotating prism doublets that use both normal dispersion and anomalous dispersion glass in order to
correct both primary and secondary chromatic aberration. The two designs perform 58% and 68%, respectively, better
than the traditional two-doublet design. We also present our design for a custom removable wide-field lens that will
allow our CCD47 to switch back and forth between an 8.6" FOV for AO science and a 25.8" FOV for acquisition.
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