Ball Aerospace has constructed a new collimator for interferometric and image quality testing of meter scale optical systems under cryogenic, vacuum conditions. Termed the Vertical Collimator Assembly (VCA), it features 1.5 m diameter off-axis parabolic and calibration flat mirrors. In order to preserve as large a volume as possible for the unit under test, the main platform is suspended inside its vacuum chamber by a hexapod, with the parabolic mirror mounted overhead. A simultaneous interferometer facilitates collimator alignment and monitoring, as well as wavefront quality measurements for the test unit. Diffusely illuminated targets may be employed for through-focus image quality measurements with pinholes and bar targets. Mechanical alignment errors induced by thermal and structural perturbations are monitored with a three-beam distance measuring interferometer to enable mid-test compensation. Sources for both interferometer systems are maintained at atmospheric pressure while still directly mounted to the main platform, reducing vibration and stability problems associated with thermal vacuum testing. Because path lengths inside the ambient pressure vessels are extremely short, problems related to air turbulence and layering are also mitigated. Inchamber support equipment is insulated and temperature controlled, allowing testing while the chamber shrouds and test unit are brought to cryogenic temperatures.
To facilitate the accurate placement and alignment of the corrective optics space telescope axial replacement (COSTAR) structure, mechanisms, and optics, the COSTAR Alignment System (CAS) has been designed and assembled. It consists of a 20-foot optical bench, support structures for holding and aligning the COSTAR instrument at various stages of assembly, a focal plane target fixture (FPTF) providing an accurate reference to the as-built Hubble Space Telescope (HST) focal plane, two alignment translation stages with interchangeable alignment telescopes and alignment lasers, and a Zygo Mark IV interferometer with a reference sphere custom designed to allow accurate double-pass operation of the COSTAR correction optics. The initial setup is configured to mimic the as-built optical axis and latch geometry of the Bay 4 position (high speed photometer) in the HST. The system is used to align the fixed optical bench (FOB), the track, the deployable optical bench (DOB), the mechanisms, and the optics to ensure that the correction mirrors are all located in the required positions and orientations on-orbit after deployment. In this paper, the layout of the CAS is presented and the various alignment operations are listed along with the relevant alignment requirements. In addition, calibration of the necessary support structure elements and alignment aids is described, including the two-axis translation stages, the latch positions, the FPTF, and the COSTAR-mounted alignment cubes.
CAS SATISFIES DEMANDING REQUIREMENTSO-8194-1245-7/931$6.UO
The Space Telescope Imaging Spectrograph (STIS) instrument is due to be installed on board the Hubble Space Telescope (HST) in 1997. STIS uses 20 filters located on a wheel that can rotate any one of 88 apertures or combination filter/aperture into the beam path. The instrument incorporates a continuous range of spectral response from the VUV (1 15.0 nm) to lum. Therefore, filters that perform in the VUV are discussed as well as filters that operate in the near infrared. Neutral density filters are also being used for on-board calibration from 300nm to Lyman-Alpha (121.6nm).
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