from the UV to the near-IR. Light en ters the spectrograph through 1 of 11 slits that range in size from 1.0 to 0.1 arc sec. Two 512-element Digicon de tectors record spectra: One measures photons from 115 to 550 nm, and the other covers 170-850 nm. Light varia tions as rapid as 40 ms are detectable with this device. Like the faint-object camera, the faint-object spectrograph contains two occulting fingers for blocking out bright sources that could overwhelm spectral information from dimmer nearby targets. Included in the faint-object spectro graph's mission are studies of the chemical composition of comets before they develop a "tail" and chemical de terminations for other galaxies. This spectrograph will also examine far-dis tant quasars that could reveal the way in which galaxies first formed and the chemical abundances of the early uni verse. High-speed photometer. Billed as the simplest with no moving parts (and, at $11 million, the "cheapest" in strument on HST), the high-speed photometer is basically a light meter for measuring light intensity, variation, and polarization. The instrument was designed and constructed at the Uni versity of Wisconsin-Madison. High speed refers to the instrument's ability to distinguish light intensity fluctua tions as rapid as every 20 μβ. Groundbased observers are limited by air tur bulence to periodic variations of 1 s. The photometer contains a GaAs photomultiplier tube and four magnet ically focused image dissector tubes—
photomultiplier-like tubes that count only the photoelectrons emitted from the region of the photocathode that is activated by incoming light. The image dissector tubes respond to a range of 120-700 nm, whereas the photomulti plier detects only red light. With expo sure times of about one-half hour, the high-speed photometer should be able to detect light sources as faint as 24th magnitude. Despite the absence of moving parts, the photometer can be operated in > 100 different modes. To accomplish this, controllers point HST so that light reaches the photometer assembly through one of four entrances (one for each dissector tube). Bolted to 3 of the entrances is a plate with 13 different interference filters arranged in a dou ble row (Figure 3). Behind each filter plate is an aperture plate with 50 dif ferent-sized holes. The precise aiming of HST sends light through a specific filter-aperture combination. All to gether, 27 different filter-aperture combinations are available for each of these three detectors. Light coming in one of the entrances also passes through a beam-splitter that sends photons to the photomultiplier tube. The fourth entrance, for polarimetry measurements, contains four near-UV filters, each backed with a polarizing filter rotated 45° from its neighbor. Fine-guidance sensors
To lock HST on target, the telescope carries three fine-guidance sensors. Only two are needed to point the satel lite, leaving the third sensor free to measure star positions (astrometry) 10
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Figure 3. High-speed photometer filter-aperture tube configuration. Depending on HST's aim, light passes through one of 13 filters and one of 50 apertures. (Courtesy of Uni versity of Wisconsin-Madison.)
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ANALYTICAL CHEMISTRY, VOL.
62, NO. 14, JULY 15, 1990 · 791 A
