Scott T. DeRigne

Practical design and performance of the stressed-lap polishing tool

We present an overview of the engineering design and empirical performance of four stressed-lap polishing tools developed at the University of Arizona. Descriptions of the electromechanical actuators, servo systems, computer interfacing, and attachment of the lap to the polishing machine are provided. The empirical performance of a representative tool is discussed in terms of accuracy, repeatability, and hysteresis. Finally, we estimate the statistical likelihood of aluminum lap-plate failure through a metal-fatigue analysis for a worst-case stress-cycling situation.

Active supports and force optimization for the MMT primary mirror

We describe the active support system and optimization of support forces for the 6.5 m primary mirror for the Multiple Mirror Telescope Conversion. The mirror was figured to an accuracy of 26 nm rms surface error, excluding certain flexible bending modes that will be controlled by support forces in the telescope. On installation of the mirror into its telescope support cell, an initial optimization of support forces is needed because of minor differences between the support used during fabrication and that in the telescope cell. The optimization is based on figure measurements made interferometrically in the vibration- isolated test tower of the Steward Observatory Mirror Lab. Actuator influence functions were determined by finite- element analysis and verified by measurement. The optimization is performed by singular value decomposition of the influence functions into normal modes. Preliminary results give a wavefront accuracy better than that of the atmosphere in 0.11 arcsecond seeing.

Toward first light for the 6.5-m MMT Telescope

Operated by the Multiple Mirror Telescope Observatory (MMTO), the multiple mirror telescope (MMT) is funded jointly by the Smithsonian Institution (SAO) and the University of Arizona (UA). The two organizations equally share observing time on the telescope. The MMT was dedicated in May 1979, and is located on the summit of Mt. Hopkins (at an altitude of 2.6 km), 64 km south of Tucson, Arizona, at the Smithsonian Institutions Fred Lawrence Whipple Observatory (FLWO). As a result of advances in the technology at the Steward Observatory Mirror Laboratory for the casting of large and fast borosilicate honeycomb astronomical primary mirrors, in 1987 it was decided to convert the MMT from its six 1.8 m mirror array (effective aperture of 4.5 m) to a single 6.5 m diameter primary mirror telescope. This conversion will more than double the light gathering capacity, and will by design, increase the angular field of view by a factor of 15. Because the site is already developed and the existing building and mount will be used with some modification, the conversion will be accomplished for only about

Fabrication and measured quality of the MMT primary mirror

20 million. During 1995, several major technical milestones were reached: (1) the existing building was modified, (2) the major steel telescope structures were fabricated, and (3) the mirror blank was diamond wheel ground (generated). All major mechanical hardware required to affect the conversion is now nearly in hand. Once the primary mirror is polished and lab-tested on its support system, the six-mirror MMT will be taken out of service and the conversion process begun. We anticipate that a 6 - 12 month period will be required to rebuild the telescope, install its optics and achieve f/9 first light, now projected to occur in early 1998. The f/5.4 and f/15 implementation will then follow. We provide a qualitative and brief update of project progress.

Machine for complete fabrication of 8-m class mirrors

The primary mirror for the Multiple Mirror Telescope Conversion is the first 6.5 m honeycomb sandwich mirror cast and polished by the Steward Observatory Mirror Lab. We describe the optical fabrication and testing of the f/1.25 paraboloid, and present the final measurements of figure accuracy and inferred image quality. Figuring was performed with a 1.2 m stressed lap--which bends under active control to match the local curvature of the optical surface--and a variety of small passive tools. The mirror was pressurized to compensate for polishing loads and thereby eliminate print-through of the honeycomb structure. The net result is a smoother surface on scales of 5 - 20 cm than has been achieved on previous honeycomb sandwich mirrors. The figure was measured with IR and visible interferometers, using refractive null correctors to compensate 810 microns of aspheric departure. The final measurements were used to calculate synthetic stellar images in a variety of seeing conditions.

Stressed-lap polishing of 3.5-m f/1.5 and 1.8-m f/1.0 mirrors

The Large Optical Generator (LOG) was originally installed as a precision generator at the University of Arizona. It has since been relocated to the Steward Observatory Mirror Laboratory, where, in addition to its tasks as generator, it can be reconfigured as a polishing machine. As such, utilizing the Mirror Labs stressed-lap techniques, LOG has recently finished a series of three 3.5 meter mirrors to high accuracy. It is currently configured as a generator for work on the 6.5 meter MMT upgrade. LOGs operating parameters and level of performance both as generator and polisher will be discussed, along with some of the unique safety features that have been built into its operation.

NGST mirror system demonstrator from the University of Arizona

The stressed-lap polishing technique has been developed to meet the challenge of polishing 8- m-class mirrors with highly aspheric figures to an accuracy consistent with the best ground- based telescope sites. The method is currently being demonstrated in the polishing of two primary mirrors, a 1.8-m f/1.0 ellipsoid and a 3.5-m f/1.5 paraboloid. The figure accuracies achieved at the time of writing are 43 nm rms surface error for the 1.8-m mirror, and 190 nm rms surface error for the 3.5-m mirror. Polishing is proceedings on both mirrors. In this paper we describe the process used for the 3.5-m mirror and the progress through the early stages of fabrication. We also summarize progress on the 1.8-m mirror.

Active supports and force optimization for a 3.5-m honeycomb sandwich mirror

Future space telescopes require primary mirrors that are much lighter than those currently being manufactured. They also must maintain optical tolerances while operating at cryogenic temperatures. We present a Mirror System Demonstrator for the Next Generation Space Telescope (NGST) that uses a thin glass facesheet with active control to achieve low mass and high surface quality. A 2-mm thick glass facesheet is controlled by miniature actuators and held together by a rigid carbon fiber frame. The 2-m diameter mirror system weighs only 13 kg/m2, including the glass, supports, actuators, support structure, and cabling. We present the status of the development and testing of this revolutionary mirror.

Initial operation of the University of Arizona NGST mirror system demonstrator

We describe the active support system and optimization of support forces for a 3.5-m honeycomb sandwich mirror. The optimization was based on interferometric figure measurements made in a vibration-isolated test tower. We obtained actuator influence functions by measurement and by finite-element analysis. The two sets of influence functions are similar in shape, but the computed figure changes are 25% smaller in magnitude than the measured figure changes. We achieved a figure accuracy of 25 nm rms surface error with the computed influence functions and only slightly worse with the measured influence functions, but were unable to reproduce the 21-nm rms surface error obtained on the passive polishing support. This implies that subtle differences between the polishing support and operational support caused small, uncorrectable figure changes. The optimization was performed by singular-value decomposition of the influence functions into normal modes. The best results were obtained using 20 - 30 out of a possible 37 modes.

Primary mirror system for the first magellan telescope

We present an update on the University of Arizona NGST (Next Generation Space Telescope) Mirror System Demonstrator (NMSD). The 2-m NMSD mirror is a lightweight space optic designed for the next generation of space telescopes. Unlike conventional monolithic mirrors, the NMSD mirror is completely active in operation. The mirror uses a thin flexible glass substrate for the optical surface and an actuated lightweight structure for surface accuracy and support. We discuss the mirror’s design and present the initial measurements of the surface figure. 2002 Optical Society of America OCIS codes: (350.6090) Space optics; (350.1260) Astronomical optics; (220.4610) Optical fabrication