Large-mirror testing facility at the National Optical Astronomy Observatories

Details Large-mirror testing facility at the National Optical Astronomy Observatories Large-mirror testing facility at the National Optical Astronomy Observatories

Sep 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991opten..30.1405c&link_type=abstract

Optical Engineering (ISSN 0091-3286), vol. 30, Sept. 1991, p. 1405-1414. Research supported by NSF and USAF.

Computer Science

13

Diffraction Patterns, Honeycomb Structures, Mirrors, Surface Properties, Telescopes, Borosilicate Glass, Fringe Multiplication, Moire Interferometry, Turbulence Effects

Scientific paper

A method for testing the surfaces of large mirrors has been developed to be used even when conditions of vibration and thermal turbulence in the light path cannot be eliminated. The full aperture of the mirror under test is examined by means of a scatterplate interferometer that has the property of being a quasi-common-path method, although any means for obtaining interference fringes can be used. By operating the test equipment remotely, the optician does not cause unnecessary vibrations or heat in the testing area. The typical test is done with a camera exposure of about a millisecond to 'freeze' the fringe pattern on the detector. Averaging up to 10 separate exposures effectively eliminates the turbulence effects. From the intensity information, a phase map of the wavefront reflected from the surface is obtained using a phase-unwrapping technique. The method provides the optician with complete numerical information and visual plots for the surface under test and the diffracted image the method will produce to an accuracy of 0.01 micron measured peak-to-valley. The method has been extensively used for a variety of test of a 1.8-m-diam borosilicate-glass honeycomb mirror, where the method was shown to have a sensitivity equal to a Foucault test.

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