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31 December 1980 Alignment And Evaluation Of The Cryogenic Corrected Infrared Astronomical Satellite (IRAS) Telescope
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Abstract
Room temperature alignment and evaluation techniques for the Infrared Astronomical Satellite (IRAS) telescope which has a primary mirror figured to correct for surface distortions at the 2K operating temperature will be discussed. Interferometric cryogenic testing of the 0.6 meter, f/1.5 lightweighted beryllium primary mirror at its intended operating temperature revealed surface distortions that could be modelled with Zernike polynomials. With this model, it was possible to derive the "inverse" of the cryo wavefront error (ideal cryo mirror) and figure the cryo correction into the primary mirror using Perkin-Elmer's Computer Controlled Polisher. It was recognized that during room temperature assembly of the system, misalignment of the secondary mirror could introduce additional unwanted aberrations that would cancel or distort the wavefront errors purposely introduced by the cryo figuring. To avoid this possible degradation and to ensure optimum telescope performance, the system Zernike polynomial co-efficients and wavefront maps generated from the in-process alignment interferograms were monitored and compared to Zernike coefficients and wavefront maps for the cryo corrected primary mirror. Performance specifications for the telescope included encircled energy on-axis and at the edge of the field of view. The predicted Zernike coefficients for the telescope at cryo temperatures were determined by subtracting the room temperature coefficients for the aligned system from the ideal cryo mirror. Encircled energy was calculated for on- and off-axis by adding the predicted Zernike coefficients to the nominal optical design prescription in the Perkin-Elmer Optical Design Computer Program. Using Zernike polynomials to iiionitor the optical quality of the telescope enabled figure and alignment errors t© be monitored, and demonstrated that the alignment tolerance was achieved. Describing the telescope's wavefront in terms of Zernike polynomials also per mitted prediction of the telescope performance for various field positions and detector or
© (1980) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Noreen Harned, Robert Harned, and Ramsey Melugin "Alignment And Evaluation Of The Cryogenic Corrected Infrared Astronomical Satellite (IRAS) Telescope", Proc. SPIE 0251, Optical Alignment I, (31 December 1980); https://doi.org/10.1117/12.959466
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