42 research outputs found
Towards optimization of pulsed sodium laser guide stars
Pulsed sodium laser guide stars (LGS) are useful because they allow for
Rayleigh blanking and fratricide avoidance in multiple-LGS systems.
Bloch-equation simulations of sodium-light interactions show that these may be
able to achieve photon returns nearly equal to, and in some cases greater than,
what is seen from continuous-wave (CW) excitation. In this work, we study the
time-dependent characteristics of sodium fluorescence, and investigate the
optimal format for the new fiber laser LGS that will be part of the upgraded
adaptive optics (AO) system on the Shane telescope at Mt. Hamilton. Results of
this analysis are examined in the context of their general applicability to
other LGS systems and the potential benefits of uplink correction are
considered. Comparisons of simulation predictions with measurements from
existing LGS are also presented and discussed.Comment: 9 pages, 7 figures, accepted by JOSA
Stroke saturation on a MEMS deformable mirror for woofer-tweeter adaptive optics
High-contrast imaging of extrasolar planet candidates around a main-sequence
star has recently been realized from the ground using current adaptive optics
(AO) systems. Advancing such observations will be a task for the Gemini Planet
Imager, an upcoming "extreme" AO instrument. High-order "tweeter" and low-order
"woofer" deformable mirrors (DMs) will supply a >90%-Strehl correction, a
specialized coronagraph will suppress the stellar flux, and any planets can
then be imaged in the "dark hole" region. Residual wavefront error scatters
light into the DM-controlled dark hole, making planets difficult to image above
the noise. It is crucial in this regard that the high-density tweeter, a
micro-electrical mechanical systems (MEMS) DM, have sufficient stroke to deform
to the shapes required by atmospheric turbulence. Laboratory experiments were
conducted to determine the rate and circumstance of saturation, i.e. stroke
insufficiency. A 1024-actuator 1.5-um-stroke MEMS device was empirically tested
with software Kolmogorov-turbulence screens of r_0=10-15cm. The MEMS when
solitary suffered saturation ~4% of the time. Simulating a woofer DM with ~5-10
actuators across a 5-m primary mitigated MEMS saturation occurrence to a
fraction of a percent. While no adjacent actuators were saturated at opposing
positions, mid-to-high-spatial-frequency stroke did saturate more frequently
than expected, implying that correlations through the influence functions are
important. Analytical models underpredict the stroke requirements, so empirical
studies are important.Comment: 16 pages, 10 figure
Integrated Laboratory Demonstrations of Multi-Object Adaptive Optics on a Simulated 10-Meter Telescope at Visible Wavelengths
One important frontier for astronomical adaptive optics (AO) involves methods
such as Multi-Object AO and Multi-Conjugate AO that have the potential to give
a significantly larger field of view than conventional AO techniques. A second
key emphasis over the next decade will be to push astronomical AO to visible
wavelengths. We have conducted the first laboratory simulations of wide-field,
laser guide star adaptive optics at visible wavelengths on a 10-meter-class
telescope. These experiments, utilizing the UCO/Lick Observatory's Multi-Object
/ Laser Tomography Adaptive Optics (MOAO/LTAO) testbed, demonstrate new
techniques in wavefront sensing and control that are crucial to future on-sky
MOAO systems. We (1) test and confirm the feasibility of highly accurate
atmospheric tomography with laser guide stars, (2) demonstrate key innovations
allowing open-loop operation of Shack-Hartmann wavefront sensors (with errors
of ~30 nm) as will be needed for MOAO, and (3) build a complete error budget
model describing system performance. The AO system maintains a performance of
32.4% Strehl on-axis, with 24.5% and 22.6% at 10" and 15", respectively, at a
science wavelength of 710 nm (R-band) over the equivalent of 0.8 seconds of
simulation. The MOAO-corrected field of view is ~25 times larger in area than
that limited by anisoplanatism at R-band. Our error budget is composed of terms
verified through independent, empirical experiments. Error terms arising from
calibration inaccuracies and optical drift are comparable in magnitude to
traditional terms like fitting error and tomographic error. This makes a strong
case for implementing additional calibration facilities in future AO systems,
including accelerometers on powered optics, 3D turbulators, telescope and LGS
simulators, and external calibration ports for deformable mirrors.Comment: 29 pages, 11 figures, submitted to PAS
Optical design for the narrow field infrared adaptive optics system (NFIRAOS) petite on the thirty meter telescope
We describe an exploratory optical design for the Narrow Field InfraRed Adaptive Optics (AO) System (NFIRAOS) Petite, a proposed adaptive optics system for the Thirty Meter Telescope Project. NFIRAOS will feed infrared spectrograph and wide-field imaging instruments with a diffraction limited beam. The adaptive optics system will require multi-guidestar tomographic wavefront sensing (WFS) and multi-conjugate AO correction. The NFIRAOS Petite design specifications include two small 60 mm diameter deformable mirrors (DM's) used in a woofer/tweeter or multiconjugate arrangement. At least one DM would be a micro-electromechanical system (MEMS) DM. The AO system would correct a 10 to 30 arcsec diameter science field as well as laser guide stars (LGS's) located within a 60 arcsec diameter field and low-order or tip/tilt natural guide stars (NGS's) within a 60 arcsec diameter field. The WFS's are located downstream of the DM's so that they can be operated in true closed-loop, which is not necessarily a given in extremely large telescope adaptive optics design. The WFS's include adjustable corrector elements which correct the static aberrations of the AO relay due to field position and LGS distance height
Recommended from our members
Amplitude variations on the Extreme Adaptive Optics testbed
High-contrast adaptive optics systems, such as those needed to image extrasolar planets, are known to require excellent wavefront control and diffraction suppression. At the Laboratory for Adaptive Optics on the Extreme Adaptive Optics testbed, we have already demonstrated wavefront control of better than 1 nm rms within controllable spatial frequencies. Corresponding contrast measurements, however, are limited by amplitude variations, including those introduced by the micro-electrical-mechanical-systems (MEMS) deformable mirror. Results from experimental measurements and wave optic simulations of amplitude variations on the ExAO testbed are presented. We find systematic intensity variations of about 2% rms, and intensity variations with the MEMS to be 6%. Some errors are introduced by phase and amplitude mixing because the MEMS is not conjugate to the pupil, but independent measurements of MEMS reflectivity suggest that some error is introduced by small non-uniformities in the reflectivity
Swimming with ShARCS: Comparison of On-sky Sensitivity With Model Predictions for ShaneAO on the Lick Observatory 3-meter Telescope
The Lick Observatory's Shane 3-meter telescope has been upgraded with a new
infrared instrument (ShARCS - Shane Adaptive optics infraRed Camera and
Spectrograph) and dual-deformable mirror adaptive optics (AO) system (ShaneAO).
We present first-light measurements of imaging sensitivity in the Ks band. We
compare measured results to predicted signal-to-noise ratio and magnitude
limits from modeling the emissivity and throughput of ShaneAO and ShARCS. The
model was validated by comparing its results to the Keck telescope adaptive
optics system model and then by estimating the sky background and limiting
magnitudes for IRCAL, the previous infra-red detector on the Shane telescope,
and comparing to measured, published results. We predict that the ShaneAO
system will measure lower sky backgrounds and achieve 20\% higher throughput
across the bands despite having more optical surfaces than the current
system. It will enable imaging of fainter objects (by 1-2 magnitudes) and will
be faster to reach a fiducial signal-to-noise ratio by a factor of 10-13. We
highlight the improvements in performance over the previous AO system and its
camera, IRCAL.Comment: 13 pages, 5 figures, SPIE Astronomical Telescopes + Instrumentation,
Montreal 201
Performance of MEMS-based visible-light adaptive optics at Lick Observatory: Closed- and open-loop control
At the University of California's Lick Observatory, we have implemented an
on-sky testbed for next-generation adaptive optics (AO) technologies. The
Visible-Light Laser Guidestar Experiments instrument (ViLLaGEs) includes
visible-light AO, a micro-electro-mechanical-systems (MEMS) deformable mirror,
and open-loop control of said MEMS on the 1-meter Nickel telescope at Mt.
Hamilton. In this paper we evaluate the performance of ViLLaGEs in open- and
closed-loop control, finding that both control methods give equivalent Strehl
ratios of up to ~ 7% in I-band and similar rejection of temporal power.
Therefore, we find that open-loop control of MEMS on-sky is as effective as
closed-loop control. Furthermore, after operating the system for three years,
we find MEMS technology to function well in the observatory environment. We
construct an error budget for the system, accounting for 130 nm of wavefront
error out of 190 nm error in the science-camera PSFs. We find that the dominant
known term is internal static error, and that the known contributions to the
error budget from open-loop control (MEMS model, position repeatability,
hysteresis, and WFS linearity) are negligible.Comment: 16 pages, 13 figures, to appear in Proc. SPIE 2010 Vol. 7736 Adaptive
Optics Systems II, high-resolution full-color version available at
http://spiedl.org
MEMS practice, from the lab to the telescope
Micro-electro-mechanical systems (MEMS) technology can provide for deformable
mirrors (DMs) with excellent performance within a favorable economy of scale.
Large MEMS-based astronomical adaptive optics (AO) systems such as the Gemini
Planet Imager are coming on-line soon. As MEMS DM end-users, we discuss our
decade of practice with the micromirrors, from inspecting and characterizing
devices to evaluating their performance in the lab. We also show MEMS wavefront
correction on-sky with the "Villages" AO system on a 1-m telescope, including
open-loop control and visible-light imaging. Our work demonstrates the maturity
of MEMS technology for astronomical adaptive optics.Comment: 14 pages, 15 figures, Invited Paper, SPIE Photonics West 201