Bruce C. Fitz-Patrick

Adaptive optics for the 6.5 m MMT

The adaptive optics system for the 6.5 m MMT conversion telescope will be the first to compensate the aberrated wavefront at the telescopes secondary mirror. This approach has unique advantages in terms of optical simplicity, high throughput and low emissivity. We report here the present state of construction, and the results of static and dynamic performance tests of the Cassegrain optical package.

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.

ARIES: Arizona infrared imager and echelle spectrograph

ARIES, a new 1-5 micrometers camera/spectrograph, is designed to capitalize on the exceptionally low thermal background and high optical throughput offered by the f/15 adaptive secondary system being built for the upgraded 6.5m MMT. With two state-of-the-art IR arrays, ARIES will provide diffraction-limited imaging in the JHKLM atmospheric windows and also echelle, long-slit spectroscopy at resolutions of 2,000 and 30,000. ARIES will also supply global wavefront tip/tilt information to the adaptive system using cryogenic pick-off mirrors to access field stars over a 50 arcsec diameter field at wavelengths from 1-2 micrometers .

Construction and testing of the wavefront sensor camera for the new MMT adaptive optics system

This paper describes the construction and testing of the Shack-Hartmann wavefront sensor camera for the new MMT adaptive optics system. Construction and use of the sensor is greatly simplified by having the 12 X 12 lenslet array permanently glued to the detector array, obviating the need for any further realignment. The detector is a frame transfer CCD made by EEV with 80 by 80 pixels, each 24 microns square, and 4 output amplifiers operated simultaneously. 3 by 3 pixel binning is used to create in effect an array of quad-cells, each centered on a spot formed by a lenslet. Centration of the lenslet images is measured to have an accuracy of 1 micrometers rms. The maximum frame rate in the binned mode is 625 Hz, when the rms noise is 4.5-5 electrons. In use at the telescope, the guide star entering the wavefront sensor passes through a 2.4 arcsec squares field stop matched to the quall-cell size, and each lenslet samples a 54 cm square segment of the atmospherically aberrated wavefront to form a guide star image at a plate scale of 60 micrometers /arcsec. Charge diffusion between adjacent detector pixels is small: the signal modulation in 0.7 arcsec seeing is reduced by only 10 percent compared to an ideal quad-cell with perfectly sharp boundaries.

Progress report on the optical system for closed-loop testing of the Multiple Mirror Telescope adaptive secondary mirror

Steward Observatory is completing the manufacture of a deformable f/15 secondary mirror for the 6.5m Multiple Mirror Telescope conversion that will, along with the wavefront sensing system, compensate for atmospheric turbulence. A potential difficulty of an adaptive secondary mirror is the ability to verify the commanded mirror shapes of a large convex deformable surface. An optical design is presently being implemented to test the deformable mirrors closed loop control system by optically projecting an artificial star to simulate starlight in the actual telescope. The test system has been designed to verify the control system by fitting into both a laboratory test structure as well as the telescope support structure itself. The optical design relies on two wavelength computer generated holograms used to remove spherical aberration as well as aid in the alignment of the test system optics by projecting alignment patterns.

Design and realization of ancillary control loops for the MMT adaptive optics system

The adaptive optics system of the Multiple Mirror Telescope is going to realize a high speed (1 kHz bandwidth) and high order (336 actuators) wavefront correction. However, to achieve the required 0.08 arcsec pointing stability the focal point of the Shack-Hartman wavefront sensor must be kept aligned to the Cassegrain focus better than 10 micrometers in spite of the non-common path tip/tilt error due to mechanical and thermal deformation of the telescope structure. The wave-front sensor must also be rotated with high precision to keep it aligned with the deformable secondary mirror in spite of the parallactic angle correction of the telescope. Our approach is to use a feed- forward loop to eliminate the adverse effect of deformation. A fast, deterministic field bus is applied to interconnect the actuators, sensors and computers. The bandwidth (500 kbs) and latency (less than 1 ms) of the DeviceNet serial bus is adequate to support our distributed control system. The field bus architecture simplifies and standardizes the control software as well as improves the reliability of the electronics by reducing the wiring.

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.

Thermal characterization of the support plate for the 6.5-m MMT thin-shell adaptive secondary mirror

The Multiple Mirror Telescope on Mt. Hopkins will soon be upgraded to a single 6.5 m primary mirror.An adaptive- optical system, featuring a thin-shell, adaptive-secondary mirror with 330 voice-coil actuators is being developed for this new telescope. The thin-shell mirror is supported by a thick, concave aluminum substrate which also holds the actuator control and monitoring electronics and serves as the reference surface. With the actuator electronics dissipating heat into the substrate, the thermal behavior of the aluminum reference plate becomes as important issue. This paper presents results form tow experiments designed to determine the thermal behavior of the reference plate.

Design of the first-generation wavefront sensor and actuator geometry for the 6.5-m MMT adaptive optical system

A description is given for the geometry of the first generation adaptive-optical (AO) system being developed for the 6.5m single-mirror telescope that will replace the Multiple Mirror Telescope on Mt. Hopkins. The AO system consists of an adaptive secondary mirror with 330-actuators and a wave front senor composed of a 13 by 13 square subaperture array. The deformable mirror actuator vibrational modes were determined from a finite element model of the adaptive-secondary mirror. Numerical simulations in based on the finite element results were carried out to determine the expected performance of the system. Two reconstruction algorithms were compared - a least squares reconstructor and a modal equalization technique developed for this AO geometry. Strehl ratios are reported for the two algorithms for various guide star magnitudes and number of corrected wave front modes.