Warren B. Davison

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.

Development of surface metrology for the Giant Magellan Telescope primary mirror

The Giant Magellan Telescope achieves 25 meter aperture and modest length using an f/0.7 primary mirror made from 8.4 meter diameter segments. The systems that will be used for measuring the aspheric optical surfaces of these mirrors are in the final phase of development. This paper discusses the overall metrology plan and shows details for the development of the principal test system - a system that uses mirrors and holograms to provide a null interferometric test of the surface. This system provides a full aperture interferometric measurement of the off-axis segments by compensating the 14.5 mm aspheric departure with a tilted 3.8-m diameter powered mirror, a 77 cm tilted mirror, and a computer generated hologram. The interferometric measurements are corroborated with a scanning slope measurement from a scanning pentaprism system and a direct measurement system based on a laser tracker.

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

Measurements of large optical surfaces with a laser tracker

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.

Support of large borosilicate honeycomb mirrors

Surface measurements represent a significant part of the cost for manufacturing large aspheric optics. Both polished and rough ground surfaces must be measured with high precision and spatial resolution. We have developed a system that couples a commercial laser tracker with an advanced calibration technique and a system of external references. This system was built to measure the off-axis primary mirror segments for the Giant Magellan Telescope where it will guide loose abrasive grinding and initial polishing. The system is further expected to corroborate the optical interferometric tests of the completed mirrors, in several low-order aberrations. The design, analysis, calibration, and measured performance of this system will be presented.

20 and 30 m telescope designs with potential for subsequent incorporation into a track-mounted pair (20/20 or 30/30).

This paper will describe and discuss the methods which are being developed to support the large borosilicate honeycomb mirrors from the Steward Observatory Mirror Lab which are being used in the MMT 6.5 m conversion and the Large Binocular Telescope. The technique is similar to previous work carried out for the 3.5 m Phillips Lab mirror support.

Manufacture of a combined primary and tertiary mirror for the Large Synoptic Survey Telescope

Any future giant ground-based telescope must, at a minimum, provide foci for seeing-limited imaging over a wide field and for diffraction-limited imaging over ~1 arcminute fields corrected by adaptive optics (AO). While this is possible with a number of design concepts, our choices are constrained if we anticipate wanting to later add a second telescope for imaging with still higher resolution, and very high contrast imaging for exoplanet studies. This paper explores designs that allow for such future development. Higher resolution imaging by interferometric combination of the AO-corrected fields of two telescopes is possible without loss of point-source sensitivity or field of view, as long as the baseline can be held perpendicular to the source and can be varied in length. This requirement is made practical even for very large telescopes, provided both can move continuously on a circular track. The 20/20 telescope illustrates this concept. Telescopes so mounted can additionally be operated as Bracewell nulling interferometers with low thermal background, making possible the thermal detection of planets that would have been unresolvable by a single 20 m aperture. In practice, limits set by funding and engineering experience will likely require a single 20 or 30 m telescope be built first. This would be on a conventional alt-az mount, but it should be at a site with enough room for later addition of a companion and track. In anticipation of future motion it should be compact and stiff, with a fast primary focal ratio. We envisage the use of large, highly aspheric, off-axis segments, manufactured using the figuring methods for strong aspherics already proven for 8 m class primaries. A compact giant telescope built under these guidelines should be able to perform well on its own for a broad range of astronomical observations, with good resistance to wind buffeting and simple alignment and control of its few, large segments. We compare here configurations with adjacent hexagonal segments and close-packed circular segments. For given segment parent size and number, the largest effective aperture is achieved if the segments are left as circles, when also the sensitivity and resolution for diffraction-limited operation with AO is higher. Large round segments can also be individually apodized for high-contrast imaging of exoplanets with the entire telescope-for example 8.4 m segments will yield 10-6 suppression 0.05 arcsec from a star at 1 μm wavelength, and at 0.25 arcsec at 5 μm.

Fabrication and measured quality of the MMT primary mirror

The Large Synoptic Survey Telescope uses a unique optomechanical design that places the primary and tertiary mirrors on a single glass substrate. The honeycomb sandwich mirror blank was formed in March 2008 by spin-casting. The surface is currently a paraboloid with a 9.9 m focal length matching the primary. The deeper curve of the tertiary mirror will be produced when the surfaces are generated. Both mirrors will be lapped and polished using stressed laps and other tools on an 8.4 m polishing machine. The highly aspheric primary mirror will be measured through a refractive null lens, and a computer-generated hologram will be used to validate the null lens. The tertiary mirror will be measured through a diffractive null corrector, also validated with a separate hologram. The holograms for the two tests provide alignment references that will be used to make the axes of the two surfaces coincide.

The Giant Magellan Telescope (GMT) structure

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.