Showing papers by "Oswald H. W. Siegmund published in 1996"

Stray light, radiometric, and spectral characterization of UVCS/SOHO: laboratory calibration and flight performance

Abstract: The Ultraviolet Coronagraph Spectrometer is one of the instruments on board the Solar and Heliospheric Observatory spacecraft, which was launched in December, 1995. The instrument is designed to make ultraviolet spectrometric measurements and visible polarimetric measurements of the extended solar corona. Prior to launch laboratory measurements were carried out to determine system level values for many of the key performance parameters. Further measurements on instrument performance have been carried out since launch. Presented are descriptions of measurement techniques and representative results.

Design and predicted performance of the Far Ultraviolet Spectroscopic Explorer

Abstract: The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite will obtain high spectral resolving power ((lambda) /(Delta) (lambda) equals 30,000) measurements of astrophysical objects in the 905 - 1195 angstroms wavelength region from low-earth orbit The instrument's high effective area (30 - 100 cm2) and low detector background will permit observations of solar system, galactic, and extragalactic targets that have been too faint for previous instruments at this high resolution The instrument design achieves both high resolution and high throughput by using four nearly identical optical channels The optics consist of four normal incidence mirrors, four high density holographically-ruled diffraction gratings, and a pair of large format double delay line detectors These components are supported by a graphite-composite structure A commercially-procured spacecraft provides pointing stability of 05 arcseconds (1 (omega) ), by using data from a Fine Error Sensor included in the instrument In early 1995 the FUSE mission was reconstructed to be a lower-cost, PI-class mission The construction phase began in December, 1995, and launch is scheduled for late 1998 We present a description of the FUSE instrument, including details of the optical and mechanical design, along with an estimate of its on-orbit performance© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

Progress in soft x-ray and UV photocathodes

Abstract: We present investigations on the improvements in quantum detection efficiency (QDE) of microchannel plate (MCP) detectors resulting from the application of various photocathode materials. Nine different photocathode materials were deposited and their QDE measured in the soft x-ray and UV region from 12 angstrom to 1850 angstrom. Four of these materials (CsCl, RbCl, RbI, BaCl) significantly enhance the QDE performance of bare MCPs, and five materials were proven unsatisfactory (AgCl, LiCl, LiI, LiF, MgBr). CsCl has very high (greater than 90%) short wavelength QDE and both CsCl and RbI have UV QDE in the region of 40%. Our studies also include life testing of a KBr photocathode for a period of over 5 years. This shows good stability, and the angular response and photoemissive characteristics over time are described. The effects of long wavelength QDE activation of KBr by exposure to 2537 angstrom photons are discussed.

Performance of small-pore microchannel plates

Abstract: Small pore size microchannel plates (MCPs) are needed to satisfy the requirements for future high resolution small and large format detectors for astronomy. MCPs with pore sizes in the range 5 micrometer to 8 micrometer are now being manufactured, but they are of limited availability and are of small size. We have obtained sets of Galileo 8 micrometer and 6.5 micrometer MCPs, and Philips 6 micrometer and 7 micrometer pore MCPs, and compared them to our larger pore MCP Z stacks. We have tested back to back MCP stacks of four of these MCPs and achieved gains greater than 2 multiplied by 107 with pulse height distributions of less than 40 percent FWHM, and background rates of less than 0.3 events sec-1 cm-2. Local counting rates up to approximately equals 100 events/pore/sec have been attained with little drop of the MCP gain. The bare MCP quantum efficiencies are somewhat lower than those expected, however. Flat field images are characterized by an absence of MCP fixed pattern noise.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Soft x-ray to FUV measurements of the grazing incidence BRDF for a selection of low-reflectance surfaces

Abstract: On future astronomical instruments for the soft x-ray to FUV, stray light may be a significant cause of background events. Currently, we are engaged in an ongoing program to identify materials that are suitable for use as low- reflectance surfaces in space based instruments. As a result, we have measured the scattering performance in this spectral region, of wide a selection of low-reflectivity materials, produced with a range of processes. We present preliminary measurements of the absolute bidirectional reflectance distribution function (BRDF) for a selection of seven of these materials. Measurements were obtained at a five spectral lines, including strong geocoronal lines, over the wavelength range 44 to 1216 angstrom at near grazing incidence. We find that in most cases for constant incident and scatter angles, the total variation of BRDF with wavelength over this range is only a factor of order ten. We also find that although we have identified materials which in many instances have lower reflectances than bead blasted aluminum, it is still a good choice for most applications given its low cost and convenience.