Measurement of large convex aspheres

被引:29
作者
Burge, JH
机构
来源
OPTICAL TELESCOPES OF TODAY AND TOMORROW: FOLLOWING IN THE DIRECTION OF TYCHO BRAHE | 1997年 / 2871卷
关键词
optical testing; interferometry; computer-generated holograms; profilometry;
D O I
10.1117/12.269059
中图分类号
O43 [光学];
学科分类号
070207 ; 0803 ;
摘要
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.
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收藏
页码:362 / 373
页数:2
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