Measurement of large convex aspheres

James H. Burge

doi:10.1117/12.269059

21 March 1997 Measurement of large convex aspheres

James H. Burge

Proceedings Volume 2871, Optical Telescopes of Today and Tomorrow; (1997) https://doi.org/10.1117/12.269059
Event: Optical Telescopes of Today and Tomorrow, 1996, Landskrona/Hven, Sweden

Abstract

Large convex aspheres 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 convex secondary mirrors that are larger, more aspheric, and more accurately figured than those for existing telescopes. Two powerful measurement techniques have been implemented at the Mirror Lab and demonstrated to be accurate and economical. The polished surfaces are interferometrically measured using holographic test plates. This measurement technique uses full-aperture test plates with computer-generated holograms (CGH) fabricated onto spherical reference surfaces. 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. This test has been implemented for secondaries up to 1.15-m diameter, with 4 nm rms surface measurement accuracy. In addition to this test, a swing arm profilometer was built to measure the rough surface during aspherization and loose abrasive grinding. The machine uses simple motions and high quality components to achieve 50 nm rms measurement accuracy over 1.8-m mirrors.

Citation Download Citation

James H. Burge "Measurement of large convex aspheres", Proc. SPIE 2871, Optical Telescopes of Today and Tomorrow, (21 March 1997); https://doi.org/10.1117/12.269059

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