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A Brief Description of the Instrument and Its Operation

The dispersers are mounted on a rotating carrousel, together with several plane mirrors used for acquisition. The first-order gratings are designated as G140L, G140M, G160M, G200M, and G270M, where "G" indicates a grating, the number indicates the blaze wavelength (in nm), and the "L" or "M" suffix denotes a "low" or "medium" resolution grating, respectively. The GHRS medium resolution first-order gratings are holographic in order to achieve very high efficiency within a limited wavelength region. G140L is a ruled grating. The first two first-order gratings, G140L and G140M, have their spectra imaged by mirror Cam-A onto detector D1, which is optimized for the shortest wavelengths. The other three gratings have their spectra imaged by Cam-B onto detector D2, which works best at wavelengths from about 1700 to 3200 Å, but which is also useful down to 1200 Å.

The carrousel also has an echelle grating. The higher orders are designated as mode Ech-A, and they are imaged onto D1 by the cross-disperser CD1. The lower orders are designated as mode Ech-B, and they are directed to D2 by CD2. Finally, mirrors N1 and A1 image the apertures onto detector D1, and mirrors N2 and A2 image onto D2. The "N" mirrors are "normal," i.e., unattenuated, while the "A" mirrors ("attenuated") reflect a smaller fraction of the light to the detectors, so as to enable the acquisition of bright stars. (The mode designated as N1 actually uses the zero-order image produced by grating G140L.)

Use of the various gratings or mirrors in concert with the camera mirrors produces one of three kinds of image at the camera focus: 1) an image of the entrance aperture, which may be mapped to find and center the object of interest; 2) a single-order spectrum; or 3) a cross-dispersed, two-dimensional echelle spectrum.

The flux in these images is measured by photon-counting Digicon detectors, and the portion of the image plane that is mapped onto the Digicon is determined by magnetic deflection coils. The detectors are the heart of the GHRS and they involve subtleties that must be understood if the instrument is to be used competently.

First, there are two Digicons: D1 and D2. D1 has a cesium iodide photocathode on a lithium fluoride window; that makes D1 effectively "solar-blind," i.e., the enormous flux of visible-light photons that dominate the spectrum of most stars will produce no signal with this detector, and only far-ultraviolet photons (1060 to 1800 Å) produce electrons that are accelerated by the 23 kV field onto the diodes. D2 has a cesium telluride photocathode on a magnesium fluoride window. Each Digicon has 512 diodes that accumulate counts from accelerated electrons. 500 of those are "science diodes," plus there are "corner diodes" and "focus diodes" (see Chapter 6).

Second, both photocathodes have granularity - irregularities in response - of about 0.5% (rms) that can limit the S/N achieved, and there are localized blemishes that produce irregularities of several percent. The Side 1 photocathode also exhibits "sleeking," which is slanted, scratch-like features that have an amplitude of 1 to 2% over regions as large as half the faceplate. The effects of these irregularities could in principle be removed by obtaining a flat field measurement at every position on the photocathode, but that is, in general, impractical. Instead, the observing strategy is to rotate the carrousel slightly between separate exposures and so use different portions of the photocathode. This procedure is called an FP-SPLIT, and with it each exposure is divided into two or four separate-but-equal parts, with the carrousel moving the spectrum about 5.2 diode widths each time in the direction of dispersion. These individual spectra can be combined together during the reduction phase.

Third, the diodes in the Digicons also have response irregularities, but these are very slight. The biggest effect is a systematic offset of about 1% in response of the odd-numbered diodes relative to the even-numbered ones. This effect can be almost entirely defeated by use of the default COMB addition procedure. COMB addition deflects the spectrum by an integral number of diodes between subexposures and has the additional benefit of working around dead diodes in the instrument that would otherwise leave image defects.

Fourth, the Digicons' diodes are about the same width as the FWHM of the point spread function (PSF) for HST. Thus the true resolution of the spectrum cannot be realized unless it is adequately sampled. That is done by making the magnetic field move the spectrum by fractions of the width of a diode, by either half- or quarter-diode widths, and then storing those as separate spectra in the onboard memory. These are merged into a single spectrum in the data reduction phase. The manner in which this is done is specified by the STEP-PATT parameter, described in more detail later. The choice of STEP-PATT also determines how the background around the spectrum is measured.

Defaults exist for these parameters and they have been set to yield the best quality of spectrum for the configuration to which they apply (except for FP-SPLIT, which must be invoked explicitly). Details on the defaults are provided later (``Accumulation Mode'' on page 50), but we strongly encourage you to use the defaults unless there are compelling reasons not to.