David S. Anderson

Full-aperture interferometric test of convex secondary mirrors using holographic test plates

Convex secondary mirrors are notoriously difficult to fabricate because of the tremendous cost and difficulty of making accurate measurements of the optical surfaces. The new 6.5- and 8-m-class telescopes require secondary mirrors that are larger, more aspheric, and more accurately figured than those for existing telescopes. The challenge of measuring these giant optics has spurred the development of a new measurement technique using holographic test plates. This test uses a full-aperture test plate with a computer-generated hologram fabricated onto the spherical reference surface. When supported a few millimeters from the secondary and properly illuminated with laser light, an interference pattern is formed that shows the secondary surface errors. The hologram consists of annular rings of metal drawn onto the curved test plate surface using a custom-built writing machine. The accuracy of the surface measurement using this technique is expected to be 35 nm P-V and 6 nm rms for a 1.65-m secondary mirror for the MMT. Considerably higher accuracy is expected for less aspheric surfaces.

Progress in the stressed-lap polishing of a 1.8-m f/1 mirror

We are in the process of polishing a 1.8-rn f/i ellipsoid with an actively stressed lap. As a preliminary exercise, we have polished the mirror as a sphere using a rigid subdiameter lap. The overall surface error was 25 nm rms, and the surface met a specification corresponding to i/8-arcsec image quality. A stressed lap 600 mm in diameter was designed and built to polish the mirror as an f/i ellipsoid. It consists of an aluminum disk which changes shape continuously under the influence of 12 moment-generating actuators. These actuators are programmed to produce the shape changes necessary to make the lap fit the mirror surface as it moves across that surface and rotates. In this paper we describe the principles and design of the lap, test results, and progress to date in polishing the 1.8-rn mirror.

Swing-arm profilometry of aspherics

A profilometer is described that utilizes the swing-arm geometry to provide surface profile measurements of large, highly aspheric surfaces. The profilometer measurement is shown to be robust against stiffness and alignment induced errors in the probe motion.

Null test optics for the MMT and Magellan 6.5-m f/1.25 primary mirrors

The instruments used to interferometrically measure the optical surfaces of the 6.5-m f/1.25 primary mirrors for the MMT conversion and Magellan Telescopes must compensate over 800 micrometers surface departure from the best fitting sphere. The errors in the optical test must not contribute more than 0.04 arc seconds FWHM to the final image and the conic constant must be held to 0.01%. This paper presents the design, analysis, fabrication, and certification of the instruments used to measure these giant mirrors to such high accuracy.

Measurement of a convex secondary mirror using a holographic test plate

A 26-cm diameter aspheric convex secondary mirror was successfully measured using a holographic test plate. This measurement demonstrates the viability of the holographic test plate method for measuring convex aspheres. An opticai writer was built and used to demonstrate the ability to write precise holograms on large, curved substrates. The hologram written for this test was fabricated using a thermochemical method that does not require the use of photoresist. The accuracy of the holographic test is demonstrated with a comparison with data from an independent Hindle test.

Optical Testing With Large Liquid Flats

We recently had two occasions to qualify optical flats of moderate size. Because of the mounting geometry, it was convenient to try measuring them with a Fizeau test against a liquid surface. In a preliminary literature search, we found few writings on the application of liquid flats. In this report, we briefly review the literature before describing our experiences and results.

Rapid fabrication strategies for primary and secondary mirrors at Steward Observatory Mirror Laboratory

The pursuit of economical fabrication of large (8 m) fast (< f/2), astronomical optics has led to the development of efficient fabrication and testing methods at the Mirror Lab. These methods rely on a mix of advanced technology blended with some traditional practices. Two fabrication strategies have been developed, one for primary mirrors and one for secondary mirrors. Both of these plans rely heavily on the use of the stressed lap both as a grinder as well as for polishing. For secondary fabrication novel methods of testing the convex, severely aspheric mirrors are used.

Optical metrology for two large highly aspheric telescope mirrors

We describe a relatively simple, but highly effective, approach to the system design and alignment of an all-refractive Offner null corrector and phase-measuring Shack cube interferometer. In addition we outline procedures for fabricating and testing the optical components. Allowable errors for all parameters are determined by a tolerance analysis that separates axisymmetric and residual figure errors. An open construction optics frame provides a high degree of metering flexibility by incorporating simple kinematic mounts that provide adjustment of each lens while also allowing the lens to be removed and replaced with <2microm absolute repeatability. Nonaxisymmetric alignment errors are removed by rotating the optics on a high-precision bearing. Axial spacings are measured with contact transducers attached to both ends of an Invar metering rod. Two completed systems have guided the stressed-lap polishing of 1.8-m f/ 1.0 and 3.5-m f/ 1.5 aspheric mirrors.

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

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.

Techniques for optical fabrication of a 2-mm-thick adaptive secondary mirror

We describe the development of techniques for the optical fabrication of the adaptive secondary mirror for the 6.5-m MMT Conversion Project. The f/15 secondary is 640 mm in diameter and consists of a 2-mm-thick convex mirror supported on 320 actuators. This mirror will be polished using the stressed lap method and measured using the holographic test plate system developed at the Mirror Lab, but it presents unique challenges because of its flexibility. During fabrication, the support of the thin mirror must be uniform and stiff enough to keep it from bending significantly under polishing forces which are 25 - 50 times the weight of the mirror. We plan to support the thin mirror by attaching it to a rigid glass substrate, and are pursuing two experimental approaches to the attachment: optical contact and blocking with pitch. The experiments are being performed by fabricating 200-mm concave prototypes of the secondary mirror.