Shear/defocus sensitivity of the mid-infrared channel (MIR) of the composite infrared spectometer (CIRS) for the Cassini mission to Saturn

John G. Hagopian; Patricia A. Losch; Julie A. Crooke; Brent Mott; Anthony J. Martino

doi:10.1117/12.323744

17 September 1998 Shear/defocus sensitivity of the mid-infrared channel (MIR) of the composite infrared spectometer (CIRS) for the Cassini mission to Saturn

John G. Hagopian, Patricia A. Losch, Julie A. Crooke, Brent Mott, Anthony J. Martino

Author Affiliations +

Proceedings Volume 3435, Cryogenic Optical Systems and Instruments VIII; (1998) https://doi.org/10.1117/12.323744
Event: SPIE's International Symposium on Optical Science, Engineering, and Instrumentation, 1998, San Diego, CA, United States

Abstract

The Composite Infrared Spectrometer (CIRS) of the Cassini mission to Saturn has two interferometers covering the far infrared and mid infrared wavelength region. The mid infrared wavelength interferometer has a focal plane consisting of a germanium focus lens and HgCdTe array. System level calibration of the CIRS Flight Unit indicated a discrepancy between the expected and actual signal levels. Testing on the CIRS breadboard and Engineering Unit indicated that defocus of the germanium lens could significantly reduce the modulation efficiency of the interferometer in the presence of a moderate degree of wavefront shear. Defocus of the lens in the focal plane was of concern because of the temperature dependence of the index of refraction of germanium and the nominal operation temperature of 170 K. The shear/defocus interaction was extensively investigated and correlated to a newly developed analytical model. It was eventually determined that the CIRS instrument was in focus, had no appreciable wavefront shear and was operating near theoretical limits. The shear/defocus effect is however, of considerable interest, since it has not been described in previous literature on interferometers.

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John G. Hagopian, Patricia A. Losch, Julie A. Crooke, Brent Mott, and Anthony J. Martino "Shear/defocus sensitivity of the mid-infrared channel (MIR) of the composite infrared spectometer (CIRS) for the Cassini mission to Saturn", Proc. SPIE 3435, Cryogenic Optical Systems and Instruments VIII, (17 September 1998); https://doi.org/10.1117/12.323744

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