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34.2 Instrument Description

The GHRS was one of the first-generation science instruments aboard HST. The spectrograph was designed to provide a variety of spectral resolutions, high photometric precision, and excellent sensitivity in the wavelength range 1100 to 3200 Å. The GHRS was a modified Czerny-Turner spectrograph with two science apertures (large: LSA, and small: SSA), two detectors (D1 and D2), several dispersers, and camera mirrors. There were also a wavelength calibration lamp, flatfield lamps, and mirrors to acquire and center objects in the observing apertures.

Schematics of the mechanical and optical layout are shown below and can be found in the GHRS Instrument Handbook. 6.0, Figures 6-1 to 6-9. The GHRS was installed as one of the axial scientific instruments, with the entrance aperture adjacent to FGS 2 and FGS 3. With the installation of COSTAR, the entrance apertures were at the former position of the High Speed Photometer. The GHRS had two science apertures, designated Large Science Aperture (LSA or 2.0) and Small Science Aperture (SSA or 0.25). The 2.0 and 0.25 designations were the pre-COSTAR size of the apertures in arcseconds. The post-COSTAR size of the LSA was 1.74 arcsec square and the SSA was 0.22 arcsec square. The separation of the two apertures was approximately 3.7 arcsec on the sky and 1.05 mm on the slit plate. The LSA had a shutter to block light from entering the spectrograph when the SSA was being used, while the SSA was always open. Because of this, scattered light from a bright target in the SSA could contaminate a wavelength calibration exposure (wavecal). The locations of the GHRS apertures relative to the spacecraft axes are displayed in Figure 34.1.

Figure 34.1: Aperture Locations Relative to Spacecraft Axes

Light from a target was collected by HST and focused onto one of the two GHRS apertures. After passing through an aperture, the light struck the collimating mirror and was directed toward the grating carrousel. The collimated beam illuminates one of the gratings or a flat mirror; selection of a grating or mirror was performed by rotating the carrousel. Camera mirrors focused the dispersed light onto one of the Digicon photocathodes. Photoelectrons from the Digicons were focused onto a linear silicon diode array.

Figure 34.2: GHRS Optics

34.2.1 Dispersers

The dispersers were mounted on a rotating carrousel, together with several plane mirrors used for acquisition. The first-order gratings were 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 were holographic in order to achieve very high efficiency within a limited wavelength region. G140L is a ruled grating.

Side 1 first-order gratings, G140L and G140M, had their spectra imaged by mirror Cam-A onto detector D1, which had a cesium iodide photocathode and was best for the shortest wavelengths (about 1050 to 1700 Å.). The other three (Side 2) gratings had their spectra imaged by Cam-B onto detector D2, which had a cesium telluride photocathode and worked best at wavelengths from about 1700 to 3200 Å, but which was also useful down to 1200 Å. Also note that detector D2 had a MgF2 front plate, which attenuated severely below Lyman-, but that detector D1 had a LiF faceplate for better throughput at the shortest wavelengths. Note that little or no flux below 1150 Å was reflected by the COSTAR mirrors because of their magnesium fluoride coatings. Figure 34.3 shows the GHRS acquisition optics; the main acquisition mirror is N2. The useful spectral wavelength ranges of the first order gratings are listed in Table 34.1.

Figure 34.3: GHRS Acquisition Optics


Useful Range (Å)

Å per diode

Bandpass (Å)














2nd order overlap above 2300 Å





2nd order overlap above 2300 Å





2nd order overlap above 3300 Å

Echelle A




Echelle B




Useful Wavelength Ranges for First-Order and Echelle Gratings

The carrousel also had an echelle grating. The higher orders were designated as mode Ech-A, and they were imaged onto D1 by the cross-disperser CD1. The lower orders were designated as mode Ech-B, and they were directed to D2 by CD2. The useful spectral range of the echelle gratings are provided in Table 34.1. Only a single order was recorded in a single observation.

Finally, mirrors N1 and A1 imaged the apertures onto detector D1, and mirrors N2 and A2 imaged 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 used the zero-order image produced by grating G140L.)

Bright targets were acquired with one of the attenuated mirrors, A1 or A2. These acquisitions took substantially longer than if the N1 or N2 mirrors were used. SSA acquisitions (ACQ/PEAKUPs) with the A1 mirror were usually doubled up to center the target in the SSA. Acquiring a target in the SSA first requires a LSA acquisition (3 x 3 search) followed by a SSA acquisition (5 x 5 search).

The GHRS LSA was 74 arcseconds from the FOS Blue aperture. Starting with Cycle 5, it became possible to acquire a faint target with the FOS and slew the telescope to position the target into the GHRS LSA. It also became possible to acquire a bright target with the GHRS and slew the telescope to position the target into the FOS Blue aperture. Therefore, a GHRS proposal executed during Cycles 5 and 6 may contain FOS observations (which you would have to request from the archive separately). This can be seen by examining the Phase II proposal.

34.2.2 Detectors

Use of the various gratings or mirrors produces one of three kinds of GHRS data:

The Detectors and Their Diodes

There were two Digicons: D1 (also known as Side 1) and D2 (Side 2). D1 had 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 dominates 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 had a cesium telluride photocathode on a magnesium fluoride window. Each Digicon had 512 diodes that accumulated counts from accelerated electrons. 500 of those are science diodes, plus there are corner diodes and focus diodes.

The 500 science diodes were 40 x 400 microns on 50 micron centers. The focus diodes were 25 x 25 microns and there were three located on each end of the array. Two 1,000 x 100 micron diodes were used to measure background and two 1,000 x 100 micron diodes were used to monitor high energy protons. Eight diodes mapped the LSA and the SSA was 1 diode wide and 1/8 diode high.

Figure 34.4: View from the Cross-Dispersers toward the Digicon Detectors (-illustrates senses of x and y motions and of increasing wavelength)

Photocathode Granularity and FP-SPLIT Observing

Both photocathodes had granularity-irregularities in response-of about 0.5% (rms) that could limit the S/N achieved, and there were localized blemishes that produced irregularities of several percent. The Side 1 photocathode also exhibited features called "sleeking," which are 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 flatfield measurement at every position on the photocathode, but that was generally impractical. Instead, the observing strategy was 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 was 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 could be combined together during the reduction phase.

Diode Irregularities and the Use of Comb-Addition

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

Sampling and Substepping with STEP-PATT

The Digicons' diodes were about the same width as the FWHM of the point spread function (PSF) for HST. Thus the true resolution of the spectrum could not be realized unless it was adequately sampled. That was 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 were merged into a single spectrum in the data reduction phase. The manner in which this was done was specified by the STEP-PATT parameter, described in more detail in Chapter 35 (see Table 35.3). The choice of STEP-PATT also determined how the background around the spectrum was measured. Figure 34.5 shows the diode arrangement; note the six large corner diodes and six focus diodes (numbers 4, 5, and 6, for example).

34.2.3 Internal Calibration

The GHRS was built with two Pt-Ne hollow-cathode lamps to provide a rich spectrum of emission lines for accurate calibration of wavelengths. The locations of these lamps and the way in which they illuminated the spectrograph optics are illustrated in the GHRS Instrument Handbook. The apertures through which calibration exposures were made were the same size as the Small Size Aperture (SSA). Moreover, calibration aperture SC2, the one most frequently used, was offset from the SSA in the x direction (the direction of dispersion), but was aligned in the y direction (see Figure 34.1). This offset in x introduced a systematic shift in wavelength between the SSA and SC2 because light from them hit the gratings at different angles. By convention, the wavelength scale of the GHRS was calculated so as to be correct for the SSA.

These wavelength calibration lamps were also used for other internal instrumental calibrations, such as DEFCALs (deflection calibration) and SPYBALs (Spectrum-Y-Balance). Only one of these lamps was available for use after the failure of Side 1 in 1991. A wavelength calibration exposure (WAVECAL) was obtained by specifying an ACCUM in the Phase II Proposal with a target of WAVE and an aperture of SC2. (Prior to 1991, SC1 was a valid aperture designation and so older observations may show it.)

A WAVECAL is not the only way to assess the quality of the wavelength scale for observations. As will be noted in the example for R136a, a SPYBAL was obtained just before an ACCUM that used a grating for the first time. A SPYBAL was just a WAVECAL that was obtained at a predetermined carrousel position for each grating, and SPYBALs were done to align the spectrum on the diodes in the y direction (perpendicular to dispersion). A SPYBAL contains wavelength information that can be used to check for zero-point offset.

34.2.4 Side 1

The installation of the GHRS Redundancy Kit during the first servicing mission (December 1993) eliminated the risk of a Side 1 power supply failure affecting Side 2. With this risk removed, Side 1 operations were resumed during Cycle 4. These operations were nominal.

When the Side 1 carrousel was commanded through the Side 2 electronics during a Side 1 observation, the telemetry word containing the carrousel position would read the Side 2 commanded position during carrousel configuration, and the Side 1 encoder position after integration. The Side 1 GHRS data headers do contain the correct Side 1 position of the carrousel.

GHRS Side 1 observations cover the time spans April 1990 to June 1991 and February 1994 to the Second Servicing Mission.

34.2.5 COSTAR

The spherical aberration of the HST primary mirror was corrected by the first servicing mission (December 1993) with the installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) assembly. COSTAR deployed corrective reflecting optics in the optical path of the GHRS. The post-COSTAR GHRS has a different response with wavelength than the pre-COSTAR instrument. Interim sensitivity files were installed in CDBS on April 16, 1994, and updated during Cycle 4 and 5 when calibration observations became available. GHRS observations obtained early in Cycle 4 will require recalibration.1

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1 We recommend, as a rule, that archival researchers recalibrate all GHRS observations in order to take advantage of the best calibrations and knowledge that is available.

Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 01/14/98 15:43:51