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

The Far Ultraviolet Spectroscopic Explorer (FUSE), successfully launched in June 1999, is an astrophysics satellite designed to provide high resolution spectra (lambda/Delta lambda = 24,000-30,000) with large effective area (20-70 cm(2)) 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 cleanroom environment. In addition to the challenging vibration and turbulence problems this environment presented, two of the flight 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. discuss the difficulties these environmental constraints presented and summarize the mitigating action.