AAS 201st Meeting, January, 2003
Session 18. Instrumentation for Infrared and Optical Observing
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[18.08] An Aluminum Secondary Mirror for the SOFIA Telescope

E. F. Erickson, N. Kunz, C. A. Brivkalns (NASA Ames), T. M. Brown (USRA), M. Honaker (Orbital Sciences Corp.)

The secondary mirror for the SOFIA telescope is made from silicon carbide. It is 352 mm in diameter, weighs 1.8 kg, and is finished to good optical tolerances. The light weight is essential for chopping with the secondary support mechanism to suppress low frequency "sky" noise. To achieve the light weight, the back side of the mirror is structured with thin-walled pockets; the face plate is 2.3 mm thick. The material is stiff, has low density and high thermal conductance, but is brittle. The latter is a concern because failure of this mirror would render the telescope inoperable. For this reason we are designing a spare secondary mirror. The spare must match the mass and moments of inertia of the SiC mirror (to permit effective chopping), but should be more robust and much cheaper. The spare should permit continuation of much of the observing program, and since many of SOFIA's observations will be made in the far-infrared, the optical quality of the spare can be significantly lower than that of the SiC secondary. Currently it appears that a bare aluminum secondary can be made to meet the requirements, while achieving a diffraction-limited wavelength of 20 microns or less. The design details will be described and discussed in the paper. We gratefully acknowledge NASA support of this work.

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