Advanced techniques for measuring primary mirrors for astronomical telescopes.

Abstract

The optical measurement of primary mirrors for astronomical telescopes has become increasingly challenging for two reasons. The mirrors, in addition to being larger, are faster and more aspheric in order to shorten the length of the telescope, and the required accuracy of the optical surfaces is more stringent. This dissertation presents improved methods for measuring these mirrors in the laboratory to the required accuracy. The wire test and the scanning pentaprism test, which measure surface slope errors, were designed and run under computer control. The wire test was used to measure the conic constant of a 3.5-m f/1.75 primary mirror to an accuracy of ±0.003 and the scanning pentaprism test measured the conic constant of a 1.8-m f/1 primary to ±0.003. Improvements in these tests were identified that could increase the accuracy significantly. Interferometric optical testing with null correctors is widely used for measuring aspheric surfaces to high accuracy. A system-level analysis of the null test is given. The test is optimized for wavefront accuracy, imaging distortion, and measurement noise from ghost reflections and diffraction. The optical design and analysis of null correctors, including designs for testing 6.5-m f/1.25 and 8.4-m f/1.14 primary mirrors are given. Several new null corrector designs and a method for performing tolerance analysis using structure functions are given. An error in the null corrector, if not detected, would cause the primary mirror to be polished to the wrong shape. (The primary mirrors for the Hubble Space Telescope and the European New Technology Telescope were misshapen because of faulty null correctors.) A new test of null correctors is presented that uses a computer-generated hologram (CGH) to synthesize a perfect primary mirror. When the CGH is measured through the null corrector, it appears as a perfect primary mirror. Apparent surface errors in this measurement can be attributed to errors in the null corrector. A complete error analysis of this test is given. This method has been proven on null correctors for 3.5-m primary mirrors, where it measured errors as small as 5.1 nm rms and confirmed the conic constants to ±0.000078.