Walter B. Fowler

Spectrally Selective Photodetectors for the Middle and Vacuum Ultraviolet

A number of “solar blind” photosurfaces have been developed in recent years. Measurements made on phototubes having photocathodes of rubidium telluride, cesium telluride, cesium iodide, and copper iodide are described. Cathode quantum efficiencies for semitransparent cathodes of rubidium or cesium telluride range from 10−1 to 10−2 electrons/quantum in the middle and vacuum ultraviolet, with long wavelength responses of less than 10−4 electrons/quantum beyond about 3500 A. By combining “solar blind” cathodes with windows of LiF, CaF2, or fused silica, detectors with relatively flat quantum yields can be produced, marked by high sensitivities in specific ultraviolet spectral regions and by very low sensitivities at all longer wavelengths.

Compact far ultraviolet emission source with rich spectral emission 1150–3100 Å

A new compact far uv emission source suitable for laboratory or space applications is described. The source is small, rugged, lightweight, spectrally rich, and bright in the 1150-3200-A region. It has met rigid spacecraft environmental tests and also is a very useful transfer standard for absolute sensitivity calibrations of spectrometric instruments.

CCD detector for the space telescope imaging spectrograph

The space telescope imaging spectrograph (STIS) is currently being developed for in-orbit installation onto the Hubble Space Telescope in 1997, where it will cover the wavelength range from 115 to 1000 nm in a variety of spectroscopic and imaging modes. For coverage of the 305 - 1000 nm region (and backup of the 165 - 305 nm) region, STIS will employ a custom CCD detector which has been developed at Scientific Imaging Technologies (SITe; formerly Tektronix CCD Products Group). This backside-illuminated device incorporates a proprietary SITe backside treatment and anti-reflective coating to extend the useful quantum efficiency shortward of 200 nm. It also features low noise amplifiers, multi-pinned-phase implants, mini-channel implants, and four quadrant readout. The CCD is thermo-electrically cooled to an operating temperature of -80 degree(s)C within a sealed, evacuated housing with its exterior at room temperature to minimize the condensation of absorbing contaminants in orbit. It is coupled to a set of low noise, flexible, fault-tolerant electronics. Both housing and electronics are being developed by the STIS prime contractor, Ball Aerospace & Communications Group. We describe here the design features, performance, and fabrication status of the STIS CCD and its associated subsystem, along with results of radiation testing.

Bidirectional Reflectance of the Moonlit Earth

From OGO-4 airglow photometer data and computed lunar spectral irradiances at the subsatellite point, the highest radiance over clouds and lowest radiance over open ocean are examined near 3914 A, 5577 A, 5893 A, 6225 A, and 6300 A in terms of bidirectional reflectance. The results are compared to and are consistent with mathematical models of the atmosphere developed by Plass and Kattawar, and with daytime measurements from OSO-3 by Neel, Griffin, and Millard.

Proton radiation effects on multi-pinned-phased CCDs

Three Tektronix 1024 x 1024 multi-pinned-phased (MPP) charge coupled devices were irradiated with protons to obtain data on CCD performance degradation in a proton radiation environment. The devices were irradiated with a spectrum of energies up to 120 MeV, simulating the total radiation dose of a long-term space experiment. Basic parameters such as charge transfer efficiency, dark current, noise, and full well were measured before and after irradiation. A test was also performed to determine the effectiveness of various thicknesses of tantalum shielding in protecting the CCD from damage. Dark current increase and CTE degradation were the most noticeable effects of proton radiation. This paper will present the objectives, test data, and conclusions of the proton testing, and will identify future testing to be performed.

Fabrication of MgF2 and LiF windows for the Hubble Space Telescope Imaging Spectrograph

Currently under fabrication at Optovac, Inc. are two prototype test windows (MgF2 and LiF) to be used on the 75 mm UV MAMA detector tubes for the Hubble Space Telescope Imaging Spectrograph (STIS). The spatial and optical constraints of this instrument dictate that the thickness of the window materials be no greater than 2 to 3 mm to achieve a minimum 50 percent transmission at Hydrogen Lyman alpha (121.6 nm), and that the window must be domed to minimize optical aberrations and provide structural strength. The detector window has an input diameter of about 100 mm with a radius-of-curvature of 70 mm. The manufacturing processes involved in the fabrication of these windows will be discussed, as well as test programs (optical and structural) to be performed at Goddard Space Flight Center (GSFC).

Space Telescope Imaging Spectrograph detectors and ultraviolet signal-to-noise capabilities

The space telescope imaging spectrograph (STIS) was designed as a versatile spectrograph capable of maintaining or exceeding the spectroscopic capabilities of both the Goddard High Resolution Spectrograph and the Faint Object Spectrograph (FOS) over the broad bandpass extending from the UV through the visible. STIS achieves performance gains over the aforementioned first generation Hubble Space Telescope instruments primarily through the use of large a real detectors in both the UV and visible regions of the spectrum. Simultaneous spatial and spectral coverage is provided through long slit or slitless spectroscopy. This paper will review the detector design and in-flight performance. Attention will be focussed on the key issue of S/N performance. Spectra obtained during the first few months of operation, illustrate that high signal-to-noise spectra can be obtained while exploiting STISs multiplexing advantage. From analysis of a single spectrum of GD153, with counting statistics of approximately 165, a S/N of approximately 130 is achieved per spectral resolution element in the FUV. In the NUV a single spectrum of GRW + 70D5824, with counting statistics of approximately 200, yields a S/N of approximately 150 per spectral resolution element. An even higher S/N capability is illustrated through the use of the fixed pattern split slits in the medium resolution echelle modes where observations of BD28D42 yield a signal-to-noise of approximately 250 and approximately 350 per spectral resolution element in the FUV and NUV respectively.

Proton-induced noise in Space Telescope digicon

The Space Telescope (ST), which carries two UV sensitive digicons, will ?ass several times per day througn a low altitude radiation belt called the South Atlantic Anomaly (CAA). This is expected to create interference in what is otherwise anticipated to be a noise-free device. Two essential components of the digicon, the semiconductor diode array and the UV transmitting window, have been shown by us to generate noise when subjected to medium energy proton radiation, a primary component of the belt. These trapped protons, having energies ranging from 2 to 400 HeV and fluences at the digicon up to 4000 P+/sec-cn2, will pass through both the window and the diode array depositing energy in each. To evaluate the effect of these protons, we irradiated engineering test models of digicon tubes to be flown on the ST with low-flux (104 10 P+/sec-cmz) monoenergetic proton beams at the University of Maryland Cyclotron. It was shown that electron-hole pairs produced by the protons passing through the diodes or the surrounding bulk causes a background count rate exceeding previous estimates by a factor of between 5 and 10. It was also shown that these counts can occur simultaneously in the output circuits of several adjacent diodes. Pulse height spectra of these proton induced counts indicate most of the bulk related counts overlap the single photoelectron peak.