The Laser Adaptive Optics system of the 6.5 m MMT telescope has now been commissioned with Ground Layer
Adaptive Optics operations as a tool for astronomical science. In this mode the wavefronts sampled by each of five laser
beacons are averaged, leading to an estimate of the aberration in the ground layer. The ground layer is then compensated
by the deformable secondary mirror at 400 Hz. Image quality of
0.2-0.3 arc sec is delivered in the near infrared bands
from 1.2-2.5 μm over a field of view of 2 arc minutes. Tomographic wavefront sensing tests in May 2010 produced open
loop data necessary to streamline the software to generate a Laser Tomography Adaptive Optics (LTAO) reconstructor.
In addition, we present the work being done to achieve optimal control PID wavefront control and thus increase the
disturbance rejection frequency response for the system. Finally, we briefly describe plans to mount the ARIES near
infrared imager and echelle spectrograph, which will support the 2 arc min ground-layer corrected field and will exploit
the diffraction limit anticipated with LTAO.
We report closed-loop results obtained from the first adaptive optics system to deploy multiple laser guide beacons. The
system is mounted on the 6.5 m MMT telescope in Arizona, and is designed to explore advanced altitude-conjugated
techniques for wide-field image compensation. Five beacons are made by Rayleigh scattering of laser beams at 532 nm
integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a
unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement
the beacon range gate. Wavefront correction is applied with the telescope's unique deformable secondary mirror. The
system has now begun operations as a tool for astronomical science, in a mode in which the boundary-layer turbulence,
close to the telescope, is compensated. Image quality of 0.2-0.3 arc sec is routinely delivered in the near infrared bands
from 1.2-2.5 μm over a field of view of 2 arc min. Although it does not reach the diffraction limit, this represents a 3 to
4-fold improvement in resolution over the natural seeing, and a field of view an order of magnitude larger than
conventional adaptive optics systems deliver. We present performance metrics including images of the core of the
globular cluster M3 where correction is almost uniform across the full field. We describe plans underway to develop the
technology further on the twin 8.4 m Large Binocular Telescope and the future 25 m Giant Magellan Telescope.
A multi-laser adaptive optics system, at the 6.5 m MMT telescope, has been undergoing commissioning in
preparation for wide-field, partially corrected as well as narrow-field, diffraction limited science observations in
the thermal and near infrared. After several delays due to bad weather, we have successfully closed the full high
order ground-layer adaptive optics (GLAO) control loop for the first time in February 2008 using five Rayleigh
laser guide stars and a single tilt star. Characterization and automated correction of static aberrations such
as non-common path errors were addressed in May 2008. Calibration measurements in preparation for laser
tomography adaptive optics (LTAO) operation are planned for the fall of 2008 along with the start of shared-risk
GLAO science observations.
We present the results of GLAO observations with the PISCES imager, a 1 - 2.5 &mgr;m camera with a field of
view of 110 arc seconds. The status of the remaining GLAO commissioning work is also reviewed. Finally, we
present plans for commissioning work to implement the LTAO operating mode of the system.
An accurate and timely model of the atmospheric turbulence profile is an important input into the construction
of tomographic reconstructors for laser tomography adaptive optics (LTAO) and multi-conjugate adaptive optics
(MCAO) using multiple guide stars. We report on a technique for estimating the turbulence profile using the
correlations between the modal reconstructions of open-loop wavefront sensor (WFS) measurements from natural
or laser guide stars. Laser guide stars can provide an estimate of the turbulence profile along the line of sight to
any suitable science target. Open-loop WFS measurements, acquired at the MMT telescope, have been analyzed
to recover an estimate of the C2n
profile. This open-loop WFS data can be used to yield turbulence estimates in
near real-time, which can be used to update the tomographic reconstructor prior to closed-loop operation.
This method can also be applied in closed-loop, using telemetry data already captured by multi-guide star
adaptive optics (AO) systems, by computing estimates of the wavefront modal covariances from the closed-loop
WFS residual error signals and the deformable mirror (DM) actuator positions. This will be of particular value
when implemented with accurate position feedback from the AO system's DMs, rather than the input actuator
commands, as is possible with an adaptive secondary mirror. We plan the first tests of the technique with the
MMT's adaptive secondary and five Rayleigh laser guide stars.
Over the past several years, experiments in adaptive optics involving multiple natural and Rayleigh laser guide stars
have been carried out by our group at the 1.5 m Kuiper telescope and the 6.5 m MMT telescope. From open-loop data
we have calculated the performance gains anticipated from ground-layer adaptive optics (GLAO) and laser tomography
adaptive optics corrections. In July 2007, the GLAO control loop was closed around the focus signal from all five laser
guide stars at the MMT, leading to a reduction in the measured focus mode on the laser wavefront sensor by 60%. For
the first time, we expect to close the full high order GLAO control loop around the five laser beacons and a tilt star at the
MMT in October 2007, where we predict image quality of < 0.2 arc seconds FWHM in K band (λ = 2.2 μm) over a 2 arc
minute field. We intend to explore the image quality, stability and sensitivity of GLAO correction as a function of
waveband with the science instrument PISCES. PISCES is a 1-2.5 µm imager with a field of view of 110 arc seconds, at
a scale of 0.11 arc seconds per pixel. This is well matched to the expected FWHM performance of the GLAO corrected
field and will be able to examine PSF non-uniformity and temporal stability across a wide field. FGD.
We describe the conceptual design of an advanced laser guide star facility (LGSF) for the Large Binocular Telescope
(LBT), to be built in collaboration with the LBT's international partners. The highest priority goal for the facility is the
correction of ground-layer turbulence, providing partial seeing compensation in the near IR bands over a 4' field. In the
H band, GLAO is projected to improve the median seeing from 0.55" to 0.2".
The new facility will build on the LBT's natural guide star AO system, integrated into the telescope with correction by
adaptive secondary mirrors, and will draw on Arizona's experience in the construction of the first multi-laser adaptive
optics (AO) system at the 6.5 m MMT. The LGSF will use four Rayleigh beacons at 532 nm, projected to an altitude of
25 km, on each of the two 8.4 m component telescopes. Initial use of the system for ground layer correction will deliver
image quality well matched to the LBT's two LUCIFER near IR instruments. They will be used for direct imaging over
a 4'×4' field and will offer a unique capability in high resolution multi-object spectroscopy.
The LGSF is designed to include long-term upgrade paths. Coherent imaging at the combined focus of the two apertures
will be exploited by the LBT Interferometer in the thermal IR. Using the same launch optics, an axial sodium or
Rayleigh beacon can be added to each constellation, for tomographic wavefront reconstruction and diffraction limited
imaging over the usual isoplanatic patch. In the longer term, a second DM conjugated to high altitude is foreseen for the
LBT's LINC-NIRVANA instrument, which would extend the coherent diffraction-limited field to an arcminute in
diameter with multi-conjugate AO.
The MMT's five Rayleigh laser guide star system has successfully demonstrated open loop wavefront sensing for both
ground-layer and laser tomography adaptive optics (AO). Closed loop correction is expected for the first time in the
autumn of 2006. The program is moving into its second phase: construction of a permanent facility to feed AO
instruments now used with the telescope's existing natural star AO system. The new facility will preserve the thermal
cleanliness afforded by the system's adaptive secondary mirror. With the present laser power of 4 W in each of the
Rayleigh beacons, we will first offer ground-layer correction over a 2 arcmin field in J, H, and K bands, with expected
image quality routinely 0.2 arcsec or better. Later, we will also offer imaging and spectroscopy from 1.5 to 4.8 μm with
a tomographically corrected diffraction limited beam. The development of these techniques will lead to a facility all-sky
capability at the MMT for both ground-layer and diffraction-limited imaging, and will be a critical advance in the tools
necessary for extremely large telescopes of the future, particularly the Giant Magellan Telescope. We describe the
present state of system development, planned progress to completion, and highlight the early scientific applications.
Ground layer adaptive optics (GLAO) can significantly decrease the size of the point spread function (PSF) and
increase the energy concentration of PSFs over a large field of view at visible and near-infrared wavelengths. This
improvement can be realized using a single, relatively low-order deformable mirror (DM) to correct the wavefront
errors from low altitude turbulence. Here we present GLAO modeling results from a feasibility study performed
for the Gemini Observatory. Using five separate analytic and Monte Carlo models to provide simulations over the
large available parameter space, we have completed a number of trade studies exploring the impact of changing
field of view, number and geometry of laser guide stars, DM conjugate altitude and DM actuator density on the
GLAO performance measured over a range of scientific wavelengths and turbulence profiles.
We describe a simple and cost-effective concept for implementing a Ground Layer Adaptive Optics (GLAO) system on
Gemini that will feed all instruments mounted at the Cassegrain focus. The design concept can provide a GLAO
correction to any of the current or future seeing-limited optical or near-infrared Gemini instruments. The GLAO design
uses an adaptive secondary mirror and provides a significant upgrade to the current telescope acquisition-and-guide
system while reusing and building upon the existing telescope facilities and infrastructure.
This paper discusses the overall design of the GLAO system including optics, opto-mechanics, laser guide star facilities,
natural and laser guide stars wavefront sensors. Such a GLAO system will improve the efficiency of essentially all
observations with Gemini and also will help with scheduling since it virtually eliminates poor seeing.
Experiments have been carried out at the MMT telescope in June 2005 and again in April 2006 to validate open loop tomographic wavefront reconstruction using five dynamically refocused Rayleigh laser beacons (RLGS) and multiple tilt natural guide stars (NGS). Wavefront sensing in this manner is recognized as a critical precursor to the development of adaptive optics for Extremely Large Telescopes. At the MMT, wavefronts from the laser beacons are recorded by five 60-element Shack-Hartmann sensors implemented on a single CCD. A wide-field camera measures image motion from multiple field stars to calculate global tilt and distinguish effects of contributions to second order aberrations from low and high altitude turbulence. Together, the signals from these sensors are used to estimate the first 45 Zernike modes in the wavefront of a star within the LGS constellation. The reconstruction is compared off line to simultaneous wavefront measurements made of the star with a separate Shack-Hartmann sensor. We will present the results in this paper and quantify the wavefront improvement expected from tomographic adaptive optics correction.
The Giant Magellan Telescope (GMT) includes adaptive optics (AO) as an integral component of its design. Planned
scientific applications of AO span an enormous parameter space: wavelengths from 1 to 25 μm, fields of view from 1
arcsec to 8 arcmin, and contrast ratio as high as 109. The integrated systems are designed about common core elements.
The telescope's Gregorian adaptive secondary mirror, with seven segments matched to the primary mirror segments, will
be used for wavefront correction in all AO modes, providing for high throughput and very low background in the
thermal infrared. First light with AO will use wavefront reconstruction from a constellation of six continuous-wave
sodium laser guide stars to provide ground-layer correction over 8 arcmin and diffraction-limited correction of small
fields. Natural guide stars will be used for classical AO and high contrast imaging. The AO system is configured to feed
both the initial instrument suite and ports for future expansion.
A demonstration of tomographic wavefront sensing has been designed, fabricated, and tested. The last of the initial testing of the dynamic refocus system at the 61" telescope on Mt. Bigelow, Arizona is presented, along with the first results from the system after its transfer to the 6.5 m MMT on Mt. Hopkins, Arizona. This system consists of a laser beam projector, and a wavefront sensor at the telescope's Cassegrain focus. The projector transmits 5 pulsed 532 nm beams in a regular pentagon of 2 arcminutes diameter from behind the telescope's secondary mirror that in good seeing can yield sub-arcsecond beacons over a 20-30 km altitude range. The wavefront sensor incorporates a dynamic refocus unit to track each returning laser pulse, and a multiple laser beacon Shack-Hartmann wavefront sensor using a novel substitute for the traditional lenslet array. A natural guide star wavefront sensor was also fielded to collect ground-truth data to compare with wavefronts reconstructed from the laser wavefront sensor measurements. All of the subsystems were shown to work, but bad weather ended the testing before the final data could be collected.
Simultaneous wavefront measurements are planned at the 6.5 m MMT telescope of five dynamically refocused Rayleigh laser beacons (RLGS) and a bright natural star to demonstrate tomographic wavefront reconstruction. In this paper, we summarize preliminary data recorded from the five laser beacons during the first telescope run at the MMT in June 2004. Beam projection is from behind the secondary of the MMT to form a regular pentagon of beacons on the sky with a radius of 60 arcseconds around the natural star. Beacon images are recorded over a range gate from 20 to 30 km, with dynamic refocus optics in the focal plane to remove perspective elongation (Stalcup, et. al., these proceedings). Separate externally synchronized Shack-Hartmann sensors record wavefront measurements of the beacons and the star, which will yield the first 33 Zernike modes from each wavefront measurement. A linear tomographic reconstructor, implemented as a matrix multiplication of the combined Zernike modal amplitudes from all five RLGS, has been computed to estimate contributions to the atmospheric aberration in two layers at 0 and 6 km. To validate the tomographic approach, the wavefront of the natural star will be predicted by computing the sum of the aberration in the direction of the star, and the prediction compared to simultaneous measurements recorded from the star directly.
We plan to take advantage of the unprecedented combination of low thermal background and high resolution provided by the 6.5m MMT's adaptive secondary mirror, to target the 3-5 micron atmospheric window where giant exoplanets are expected to be anomalously bright. We are in the process of building a 3-5 micron camera that we will use to carry out a survey to characterize the prevalence and distribution of giant planets around nearby, Sun-like stars. Sensitivity estimates show that for a 1 Gyr old G0V primary at 10 pc, we expect to detect 5 MJupiter and 15 MJupiter exoplanets at angular separations greater than 0.45-2.1" and 0.2-1.2" respectively. Monte Carlo simulations based on these sensitivity estimates and a sample of 80 young (<1 Gyr), nearby (<20 pc)
M0V-F0V stars, predict the detection of 15±3 exoplanets with masses of 4-15 MJupiter and separations of 17-50 AU. Construction of the camera is currently underway and on-telescope testing is expected in the Fall 2004-Winter 2005.
Adaptive optics will play a crucial role in achieving the full potential of the next generation of large diameter telescopes. In this paper, we present an optical design for a multi-conjugate adaptive optics system for the Giant Magellan Telescope, a 25.7 m telescope with a primary mirror consisting of seven 8.4 m segments. The tri-conjugate MCAO optics is based on adaptive secondary technology developed for the MMT telescope and incorporates dynamic refocus optics for the laser guide star wavefront sensors. We use the results of analytic (non-Monte-Carlo) numerical
simulations to determine the optimal configuration of deformable mirrors as well as laser and natural guide stars. The simulation results are extended to include and quantify the effects of wavefront sensor and control loop delay noise as well as dynamic refocus and fitting error on the expected system performance and sky coverage.
Observations have been made at the Steward Observatory 1.55 m telescope of a four-star asterism in the constellation Serpens Cauda, using a Shack-Hartmann wavefront sensor. The stars are all within a 2 arcminute field, and range in apparent brightness from mv of 9.4 to 10.6. The instrument placed a 5 x 5 array of square subapertures across the pupil of the telescope, and had sufficient field of view to allow wavefront data to be recorded from all four stars simultaneously. Snapshots at 1/30 s exposure time were recorded, with no temporal coherence between exposures. We have reconstructed the first 20 Zernike modes from the slope data for each star. In a preliminary analysis, we show that the wavefront aberration in each star can be roughly halved by subtracting the average of the wavefronts from the other three stars. The averages represent estimates of the aberration introduced by the lowest few hundred meters of the atmosphere, so the result provides an early indication of the potential for image sharpening by compensation of boundary layer turbulence.
A single-conjugate adaptive optics (AO) system for the 6.5 m MMT based on an adaptive secondary mirror is now undergoing commissioning at the telescope. In addition to providing the basis for a program of scientific observations, the system will serve as a platform for the development of a dual-conjugate AO system in which a constellation of five refocused Rayleigh laser guide stars (RLGS) will provide the majority of the wavefront information. A SCIDAR campaign at a telescope close to the MMT site is beginning to generate explicit measurements of the atmospheric turbulence profile. In this paper, we describe the current system design, and present the results of initial performance simulations using the measured atmospheric parameters.
A handful of groups around the world are actively working on the development of the next generation of telescopes of 30 m diameter and more. Present implementations of adaptive optics will be inadequate to realize the full resolving power of these new instruments in imaging and spectroscopy. Instead, multi-conjugate adaptive optics (MCAO) systems are being contemplated. We explore here the application of MCAO using laser guide beacons, to a 30 m telescope. Using a new simulation code, we show that reliance on the expensive lasers needed to generate sodium resonance beacons can be reduced through the use of refocused Rayleigh laser guide stars at much lower cost.
We show that in the geometric optics approximation, modes of the phase distortion of order less than or equal to the number of deformable mirrors in the MCAO are correctable with no isoplanatic error. A new figure of merit is derived which predicts the relative ability of a chosen beacon/deformable mirror architecture to sense and correct wavefront aberration, based solely on knowledge of the optical geometry and the statistics of the aberration to be corrected. Numerical simulation can therefore be minimized by avoiding the exploration of unpromising beacon arrangements.