This paper will report on efforts to automatically calibrate in situ a phase-diversity (PD) wavefront sensing and control
(WFS&C) system, the results of which are demonstrated on the General Dynamics Advanced Information System's
(GDAIS') QuickStar testbed1, a dual deformable mirror (DM) system which operates at 100Hz sampling rate. The
iterative automatic calibration (AutoCal) process includes both coarse and fine calibration modes, initial closed-loop
flattening of the commercial-off-the-shelf (COTS) DMs, estimation of the system's static wavefront - including DM
print-through, determination of PD-derived actuator influence functions, formulating the resulting system matrix and the
resulting forward-model parameters. Analyses of the system after the calibration routines shows low-order WFS
accuracy of ~0.005λ RMS and closed-loop residual wavefront measurement of ~0.002λ. All of these results were
accomplished with a software package that takes on the order of one hour to operate.
A proof-of-concept phase diversity (PD) wavefront sensing and control (WFS&C) testbed has been developed that
displays 5/1,000 wave RMS accuracy, operates at a sample rate of 100Hz, uses the extended scene of interest in lieu of a
guide star, and is comprised of all low-cost commercial-off-the-shelf (COTS) parts - including the PD processor. This testbed allows closed-loop
operation via a dual deformable-mirror (DM) concept where two DMs are optically conjugate to the exit pupil: one
acting as an independent disturbance and the other reacting to PD WFS&C commands in order to correct the system
wavefront. The use of low-cost, COTS components demonstrated the flexibility of a PD-only
WFS&C approach, and additionally allowed for this system to be conceived, designed, assembled and brought to
operation in approximately nine months. Automatic calibration efforts begun on this testbed have allowed for the quick
discrimination of prominent PD forward-model parameters and a more rapid verification and validation (V&V) process.
Also aiding the V&V process is a novel spatial-heterodyning optical interferometer that collects all information in a
single snapshot and may be made synchronous with the fast PD sample rate. This demonstration proves a PD-only
WFS&C subsystem capability suitable for use on a wide variety of adaptive-optics imaging systems.
Phase Diversity (PD) is a wavefront-sensing technology that offers certain advantages in an Adaptive-Optics
(AO) system. Historically, PD has not been considered for use in AO applications because computations have
been prohibitive. However, algorithmic and computational-hardware advances have recently allowed use of PD
in AO applications. PD is an attractive candidate for AO applications for a variety of reasons. The optical
hardware required is simple to implement and eliminates non-common path errors. In addition, PD has also
been shown to work well with extended scenes that are encountered, for example, when imaging low-contrast solar
granulation. PD can estimate high-order continuous aberrations as well as wavefront discontinuities characteristic
of segmented-aperture or sparse-aperture telescope designs. Furthermore, the fundamental information content
in a PD data set is shown to be greater than that of the correlation Shack-Hartmann wavefront sensor for the
limiting case of unresolved objects. These advantages coupled with recent laboratory results (extended-scene
closed-loop AO with PD sampling at 100 Hz) highlight the maturation of not only the PD concept and algorithm
but the technology as an emerging and viable wavefront sensor for use in AO applications.
This article reports on the novel patent pending Optical Spatial Heterodyne Interferometric Fourier Transform Technique
(the OSHIFT technique), the resulting interferometer also referred to as OSHIFT, and its preliminary results. OSHIFT
was borne out of the following requirements: wavefront sensitivity on the order of 1/100 waves, high-frequency
wavefront spatial sampling, snapshot 100Hz operation, and the ability to deal with discontinuous wavefronts. The first
two capabilities lend themselves to the use of traditional interferometric techniques; however, the last two prove difficult
for standard techniques, e.g., phase shifting interferometry tends to take a time sequence of images and most
interferometers require estimation of a center fringe across wavefront discontinuities. OSHIFT overcomes these
challenges by employing a spatial heterodyning concept in the Fourier (image) plane of the optic-under-test. This
concept, the mathematical theory, an autocorrelation view of operation, and the design with results of OSHIFT will be
discussed. Also discussed will be future concepts such as a sensor that could interrogate an entire imaging system as
well as a methodology to create innovative imaging systems that encode wavefront information onto the image. Certain
techniques and systems described in this paper are the subject of a patent application currently pending in the United
States Patent Office.
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.
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.
Dynamically refocusing the Rayleigh backscatter of a modestly powered laser beacon is a concept for increasing LGS brightness by 10 times. Dynamic refocus will allow for high photon return from multiple Rayleigh beacons enabling MCAO for wide field correction of the MMT and Magellan telescopes. In a system without dynamic refocus, light from a beacon integrated from 20 to 30km is blurred to a length of 14arcsecs. In a system with dynamic refocus, the bow tie is restored to a spot limited only by atmospheric seeing. The dynamic refocus system has been designed to deliver images with <3/4arcsec of induced aberration. This paper reports on field tests performed on the Mt. Bigelow Observatory 61” telescope, optically configured to appear as an off-axis sub-aperture of the 6.5m MMT. In these tests the Rayleigh backscatter from pulses of a Q-switched doubled Nd:YAG operating at 5kHZ was dynamically refocused. These preliminary tests present an uncorrected 7 by 3arcsec beacon image. The 7arcsec length is a result of using a field stop as the range-gating mechanism and the 3arcsec limit is due to double pass imaging (projecting and imaging) through the atmosphere in less than ideal seeing conditions. Upon correction, this 7x3arcsec image is dynamically refocused to a 3arcsec FWHM diameter spot.
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.
We present the design, laboratory tests and preliminary field tests of a dynamic refocus system for 351nm Rayleigh beacon laser guide stars. The purpose of dynamic refocus is to increase the beacon signal from a pulsed laser, by maintaining focus in a fixed plane while the laser pulse travels through the atmosphere over an extended height range. The focusing element in our system is a moving concave mirror. The optics have been designed and built to focus on a ring of 5 beacons at 1 arc minute radius at the 6.5 m MMT, covering the range 18 through 40 km. Laboratory tests of image quality resulted in 0.5 arcsec refocused images corresponding to the height range 22 through 28 km, free from spherical aberration. Preliminary field tests were performed on the Mt. Bigelow Observatory 1.5 m telescope, with a frequency tripled, Q switched YLF laser beam projected from a 25 cm telescope. To simulate an off axis sub aperture of the MMT, the laser and telescope axes were set 3 m apart and reimaging optics were placed ahead of the refocus unit to image at the same plate scale as the MMT (500 μm/arcsec). Returns from different heights were selected by gating the detector with a Pockels cell. Returns over a 10 km height range from 8km to 18km were brought into focus for a total mirror motion measured to be 900 μm. The system is now ready for testing dynamic refocus, which will be accomplished by attaching the mirror to a metal resonator tuned to the laser pulse frequency. The range from 23 to 35 km to be used will require a motion of 500 μm.
We report initial results from a prototype system to generate multiple Rayleigh laser guide stars for adaptive optics from a single pulsed laser at 354 nm wavelength. A 3.2 mW laser pulsed at 630 Hz was used to project three beams on the sky simultaneously, each pulsed at 210 Hz. A spinning mirror was used to direct the pulses in three directions at the vertices of an equilateral triangle 90 arcsec across. Laser pulses were triggered by a synchronising electrical pulse from the motor. Dynamic focusing optics in the receiving telescope will in future be used to hold such beacons from more powerful lasers in focus over a height range of many kilometers. Multiple beacons can be used to derive tomographic information on the vertical distribution of the aberration. We show initial analytical and numerical work on how the unique features of refocused Rayleigh beacons can improve the tomographic wavefront measurement for multi-conjugate adaptive optics.