Todd K. Barrett

Artificial neural network for the determination of Hubble Space Telescope aberration from stellar images.

An artificial-neural-network method, first developed for the measurement and control of atmospheric phase distortion, using stellar images, was used to estimate the optical aberration of the Hubble Space Telescope. A total of 26 estimates of distortion was obtained from 23 stellar images acquired at several secondary-mirror axial positions. The results were expressed as coefficients of eight orthogonal Zernike polynomials: focus through third-order spherical. For all modes other than spherical the measured aberration was small. The average spherical aberration of the estimates was -0.299 microm rms, which is in good agreement with predictions obtained when iterative phase-retrieval algorithms were used.

Full-system laboratory testing of the F/15 deformable secondary mirror for the new MMT adaptive optics system

We will present a system to perform closed-loop optical tests of the 64 cm diameter, 336 actuator adaptive secondary made at the Steward Observatory Mirror Laboratory. Testing will include Shack-Hartmann wavefront sensing and modal correction of static and dynamic aberrated wavefronts. The test optical system is designed so that experiments can be made with both the focal plane instrument and secondary installed in their normal configuration at the MMT, or with the same 9 m spacing in a laboratory test tower. The convex secondary will be illuminated at normal incidence through two 70 cm diameter lenses mounted just below. The artificial, aberrated star is projected from near the wavefront sensor in the Cassegrain focus assembly. Computer generated holograms correct for spherical aberration in the really optics at the test wavelengths of 0.594 and 1.5 micrometers . Atmospheric turbulence is reproduced by two spinning transmission plates imprinted with Kolmogorov turbulence. The Shimmulator will give us the opportunity to test fully the adaptive optics system before installation at the new MMT, hence saving much precious telescope time.

MMT adaptive secondary mirror prototype performance

The new 6.5 m single mirror multiple mirror telescope (MMT) will be equipped with adaptive optics capabilities to enhance high resolution infrared astronomy. Before we build the 64 cm diameter adaptive secondary, we fabricated a smaller prototype mirror. The adaptive secondary uses voice coil force actuators with an average spacing of 30 mm. Surrounding each actuator is an analog capacitor position sensor operating in a digital closed loop at 10 kHz. This allows the force actuators to be controlled as if they were position actuators. The adaptive secondary configuration and performance test results are presented, followed by the changes to be incorporated into the next curved shell prototype.

Final prototype design for the adaptive secondary mirror of the 6.5-m MMT

The upgraded 6.5 m MMT in Arizona will use an adaptive secondary to optimize performance in the near infrared spectral region. The secondary mirror is a 2 mm thick, 640 mm diameter Zerodur shell suspended only by a flexible center hub. Three hundred voice coil actuators installed in an aluminum reference surface deform the shell according to commands from a wavefront sensor. Capacitor position sensors surrounding each actuator provide feedback in an inner servo loop, much faster than the exterior wavefront sensor control bandwidth. A 60 actuator prototype, nearly identical to the final adaptive secondary size, has been built and is currently being tested.

6.5-m MMT infrared adaptive optics system: detailed design and progress report

We present an overview of the new adaptive system under development for the conversion of the Multiple Mirror Telescope (MMT) to a 6.5 m continuous primary mirror. The system is optimized for diffraction-limited imaging from 1.6 to 2.2 micrometer wavelength, using an adaptive secondary mirror which directly feeds an infrared science detector at f/15 Cassegrain focus. Nearly full sky coverage will be obtained using a low-power, continuous wave (cw) sodium laser beacon to sense high-order wavefront errors, with image motion sensing using a quadrant detector sensitive to infrared field star photons in the 1.2 - 1.6 micrometer band. Components are currently under development, so that the adaptive instrument can be integrated with the new 6.5 m telescope soon after first light.

Adaptive secondary mirror for the 6.5-m MMT

We report the latest progress on the design, fabrication and testing of the adaptive secondary mirror to be used in the adaptive optics system to for the 6.5m upgrade to the Steward Observatorys MMT. The adaptive secondary will use electromagnetic force actuators in conjunction with a rigid reference structure to deform a thin and flexible glass facesheet. The facesheet is fabricated with figure accuracy comparable to the surface of a traditional static secondary mirror. The flexible facesheet can however, be deformed by the actuators to conjugate the changing atmospheric aberration. Capacitive position sensors are placed at each actuator and are used to rapidly measure the position of the glass facesheet relative to the rigid reference structure. These measurements are used as feedback in a servo control-loop which maintains the desired figure of the adaptive secondary facesheet. In the proposed design the mechanical interface between the facesheet and the reference structure is limited to a small hub in the center of the facesheet. Due to heat dissipation in the electromagnetic voice-coils a temperature control system is required to maintain the facesheet of the adaptive secondary near the ambient temperature of the atmosphere. We report on laboratory tests of a nearly full size 60 actuator adaptive secondary prototype. We include tests of actuator stroke and position accuracy, control-loop stability, and closed-loop bandwidth.

Recent results for visible Rayleigh guide-star atmospheric experiments

We report on the operation and performance of a complete integrated 1 m adaptive optics systems for compensation of atmospheric distortion of optical wavefronts. Both visible artificial laser guide stars (doubled Nd:YAG laser with wavelength of 0.532 micrometers ) and natural stars can be used as sources for reference wavefronts. A polarization shearing interferometer which uses a narrow optical bandwidth and has 500 subapertures is employed to sense wavefront distortion. These measurements are used to compute a conjugate wavefront to the distorted input light. The computed conjugate is then imprinted on a deformable mirror which consists of 500 individual square mirror segments. The effectiveness of the compensation is determined from a measured PSF of the system. Both indoor benchtop and atmospheric experiments are under way to test the performance of the integrated system. The results of these tests so far are very promising, yielding short-exposure images at 0.532 microns which contain discernible energy at the diffraction limit of 0.1 arcsec.

Recovery of atmospheric phase distortion from stellar images using an artificial neural network

We report recent experimental verification of an new method to determine atmospheric phase directly from focused images of starlight. An artificial neural network is used to infer the phase from two images of a star, one at the exact focus and another intentionally out of focus. We applied the network to images of Vega obtained on the 1.5 m telescope at Starfire Optical Range (SOR), Kirtland Air Force Base, Albuquerque, New Mexico. Neural network predictions agree well with phase reconstructions using a conventional Hartmann wavefront sensor. The network approach offers a simple, inexpensive way to implement adaptive optics on astronomical telescopes in the near term.

High-bandwidth interferometer for real-time measurement of deformable mirrors

We report on the design and fabrication of a high bandwidth interferometer suitable for real- time measurement of the figure of a deformable mirror. The design allows for measurement of mirror figure in terms of optical path differences (OPD) between the surface of the mirror and a static reference wavefront. Measurements are made on a 31 by 31 square grid. This instrument is relevant for atmospheric adaptive optics systems because it provides a method for accurately monitoring the figure of a deformable mirror during real-time compensation of atmospheric turbulence. Measured values of OPD on the mirror surface are output in digital form at approximately 10 kHz and can be used as a feedback signal in a digital control-loop for driving the deformable mirror. The system uses a common 4-bucket or 4-measurement interferometric algorithm to compute OPD. The maximum measurable OPD is +/- 7.5 waves. Tests of the completed interferometer indicate that it can routinely measure the dynamic changes in figure of an optical mirror. Preliminary tests indicate that the measurements are accurate to approximately (lambda) /25.

High-bandwidth long-stroke segmented mirror for atmospheric compensation

Segmented Adaptive Optic Mirrors have been developed, fabricated, and demonstrated in real time atmospheric compensation systems. Until recently, most Segmented Adaptive Optic Mirrors have been designed for single wavelength applications and have not required more than 1.5 (mu) of surface motion since absolute phasing of the surface is not required for very narrow bandwidth compensation. Requirements for astronomical and imaging systems have required the design and fabrication of long stroke (6 - 10 (mu) ) segmented mirrors capable of absolute phasing of the segments, optical response from 0.4 to 3.5 (mu) and bandwidths above 2.5 KHz.