Hardware and methods of the optical end-to-end test. of the Far Ultraviolet Spectroscopic Explorer (FUSE)

The Far Ultraviolet Spectroscopic Explorer (FUSE), successfully launched in June 1999, is an astrophysics satellite designed to provide high spectral resolving power (lambda/Delta lambda = 24,000-30,000) over the interval 90.5-118.7 nm. The FUSE optical path consists of four co-aligned, normal incidence, off-axis parabolic primary mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographic gratings and delay line microchannel plate detectors. We describe the hardware and methods used for the optical "end-to-end" test of the FUSE instrument during satellite integration and test. Cost and schedule constraints forced us to devise a simplified version of the planned optical test which occurred in parallel with satellite thermal-vacuum testing, The optical test employed a collimator assembly which consisted of four co-aligned, 381 mm diameter Cassegrain telescopes positioned above the FUSE instrument, providing a collimated beam for each optical channel. A windowed UV light source, remotely adjustable in three axes, was mounted at the focal plane of each collimator. Problems with the UV light sources, including high f-number and window failures, were the only major difficulties encountered during the test. The test succeeded in uncovering a significant problem with the secondary structure used for the instrument cavity and, furthermore, showed that the mechanical solution was successful. The hardware was also used extensively for simulations of science observations, providing both UV light for spectra and visible light for the fine error sensor camera.