Interferometric alignment and figure testing of large (0.5 m) off-axis parabolic mirrors in a challenging cleanroom environment

Robert H. Barkhouser; Raymond G. Ohl IV

doi:10.1117/12.369210

11 November 1999 Interferometric alignment and figure testing of large (0.5 m) off-axis parabolic mirrors in a challenging cleanroom environment

Robert H. Barkhouser, Raymond G. Ohl IV

Author Affiliations +

Proceedings Volume 3782, Optical Manufacturing and Testing III; (1999) https://doi.org/10.1117/12.369210
Event: SPIE's International Symposium on Optical Science, Engineering, and Instrumentation, 1999, Denver, CO, United States

Abstract

The Far Ultraviolet Spectroscopic Explorer (FUSE), successfully launched in June 1999, is an astrophysics satellite designed to provide high resolution spectra ((lambda) /(Delta) (lambda) equals 24,000 - 30,000) with large effective area (20 - 70 cm²) over the interval 90.5 - 118.7 nm. The FUSE instrument consists of four co-aligned, off-axis parabolic primary mirrors which focus light into separate spectrograph channels. The mirrors are rectangular (407 X 372 mm) and fabricated from lightweighted Zerodur blanks. We describe a straightforward method for aligning these off-axis parabolas in an autocollimation setup via qualitative and quantitative analysis of static interferograms. Initial alignment is achieved rapidly by visual inspection of the interferogram as adjustments are made in vertical and horizontal alignment. Fine alignment to the limit of the optical system then proceeds with small alignment steps and fringe analysis software to find the position which minimizes wavefront error. This method was used for figure testing the FUSE primary mirrors throughout build-up and qualification of the flight mirror assemblies. The far- ultraviolet reflectivity of the FUSE mirrors is very sensitive to molecular contamination. All mirror testing thus took place in a strictly controlled class 1000 clean room environment. In addition to the challenging vibration and turbulence problems this environment presented, two of the fight mirrors were coated with lithium fluoride over aluminum. This necessitated purging the setup with dry nitrogen, as the lithium fluoride coating degrades with exposure to water vapor. We discuss the difficulties these environmental constraints presented and summarize the mitigating action.

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Robert H. Barkhouser and Raymond G. Ohl IV "Interferometric alignment and figure testing of large (0.5 m) off-axis parabolic mirrors in a challenging cleanroom environment", Proc. SPIE 3782, Optical Manufacturing and Testing III, (11 November 1999); https://doi.org/10.1117/12.369210

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