25 November 2002 Comparison of stress relief procedures for cryogenic aluminum mirrors
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Abstract
The Infrared Multi-Object Spectrograph is a facility instrument for the KPNO Mayall Telescope. IRMOS is a low- to mid-resolution, near-IR (0.8-2.5 um) spectrograph that produces simultaneous spectra of ~100 objects in its 2.8 × 2.0 arcmin field of view. The instrument operating temperature is ~80 K and the design is athermal. The bench and mirrors are machined from Al 6061-T651. In spite of its baseline mechanical stress relief, Al 6061-T651 harbors residual stress, which, unless relieved during fabrication, may distort mirror figure to unacceptable levels at the operating temperature (~80 K). Other cryogenic, astronomy instruments using Al mirrors have employed a variety of heat treatment formulae, with mixed results. We present the results of a test program designed to empirically determine the best stress relief procedure for the IRMOS mirrors. Identical test mirrors are processed with six different stress relief formulae from the literature and institutional heritage. After figuring via diamond turning, the mirrors are tested for figure error at room temperature and at ~80 K for three thermal cycles. The heat treatment procedure for the mirrors that yielded the least and most repeatable change in figure error is applied to the IRMOS mirror blanks. We correlate the results of our optical testing with heat treatment and metallographic data.
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Raymond G. Ohl, Michael P. Barthelmy, Said Wahid Zewari, Ronald W. Toland, Joseph C. McMann, David F. Puckett, John G. Hagopian, Jason E. Hylan, John Eric Mentzell, Ronald G. Mink, Leroy M. Sparr, Matthew A. Greenhouse, John W. MacKenty, "Comparison of stress relief procedures for cryogenic aluminum mirrors", Proc. SPIE 4822, Cryogenic Optical Systems and Instruments IX, (25 November 2002); doi: 10.1117/12.451762; https://doi.org/10.1117/12.451762
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KEYWORDS
Mirrors

Aluminum

Heat treatments

Spherical lenses

Cryogenics

Single point diamond turning

Temperature metrology

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