5.7. Goddard High Resolution Spectrograph (GHRS)
The GHRS *(8) was one of the two original spectrographs on board HST. It was replaced during the second servicing mission. The GHRS obtained high resolution spectra between 1150 Å and 1800 Å with resolving power between R = 2000 (for the "L") gratings and 25000 (for the "M") on one of its sides; while for the other the resolving powers ranged between 25000 and 80000 and the wavelength range between 1680 Å and 3200 Å. Each of these sides had its own detector (a Digicon) with 512 diodes each. The central 500 diodes were the ones used to obtain the spectra, while the first and last six (the so-called "special diodes") collect information about the observation itself.
The GHRS had two apertures: the Large Science Aperture (LSA) with a pre-COSTAR size of 2.0" (1.74" post-COSTAR) and the Small Science Aperture (SSA) with a pre-COSTAR size of 0.25" (0.22" post-COSTAR). Almost all the light from a point source was captured by the LSA. The LSA was used to obtain precise fluxes, while for the determination of radial velocities or line profiles it was the SSA.
The seven gratings, echelles and mirrors (used for the acquisitions) are mounted on a movable carrousel. Depending on the configuration, either an image of the aperture, a one or two-dimensional spectrum was obtained.
Due to the granularities of the photocathode (that limit the signal to noise) spectra were obtained divided in two or four parts, moving slightly the carrousel each time. This procedure is called FP-SPLIT. The final spectrum was obtained by combining them.
The size of the GHRS diodes is larger than the PSF. To re-gain the actual resolution it was necessary then to oversample the spectrum, ie. to allow the light to lay over several diodes. This was done by diverting the beam a quarter or half a diode and reading the spectrum after each movement. The number of pixels in the calibrated spectrum depended how these deviations were made. The raw spectrum always has 500 (no deviations made), 1000 (0.5 diode shift) or 2000 (0.25 diode shift) pixels.
5.7.1 Gratings and echelles
The first order GHRS gratings are listed in Table 21. The width listed corresponds to the number of Å that were obtained per observation.
|Name||Range (Å)||Å/diode||Width (Å)||Side|
5.7.2 What constitutes a GHRS observation?
An "observation" with the GHRS is composed of several pairs of archives, whose name always start with Z. The header, as usual, is a text archive whose extension ends with h and the binary data's extension with d.
The different files, in GEIS format, that constitute an "observation" are briefly described in the following table. Please refer to the HST Data Handbook for more details.
Note that a calibrated "spectrum'" is composed of two pairs of files:
|d0h /d0d||uncalibrated science data|
|q0h /q0d||data quality file|
|x0h/x0d||data from the special diodes|
|d1h/d1d||acquisition data (return to brightness mode)|
|c0h /c0d||wavelength-calibrated data|
|c1h/ c1d||flux-calibrated data|
|c3h/c3d||special diodes calibrated data|
The different exposure types are described by the OBSMODE header parameter, which are shown in Table 22.
|DEFCAL||centering of the source, no spectrum||source name|
|SPYBAL||calibration lamp spectrum||WAVE|
|IMAGE||image of the photocathode||source name|
|ACCUM||spectrum count integration||source name|
|RAPID||rapid acquisition without accumulation||source name|
TARGNAME is the name with which the source is referred to in the header. If a special calibration was requested, the keywords are DATE-OBS = ACCUM and TARGNAME = WAVE.
Table 23 describes some of the header parameters of interest from a GHRS observation:
|APERTURE||aperture used (LSA, SSA)|
|RA_APER1, DECAPER1||aperture position|
|EXPOSURE||exposure time per group|
|EXPSTART||exposure time in MJD|
|EXPTIME||total exposure time|
|FP_SPLIT||FPSPLIT mode (2 or 4)|
|GCOUNT||number of groups|
|NAXIS1||number of pixels in the spectrum|
|RPTOBS||number of repeated observations|
5.7.3 Working with GHRS spectra
If the spectra were obtained with an FP-SPLIT before starting the analysis, it is necessary to combine them using the STSDAS tasks poffsets and specalign. The spectrum is still divided in two: the wavelength and flux calibrated parts. To combine them and proceed with the study, the mkmultispec task is used.
The wavelength calibration has an RMS error of 1.25 diodes. If additional data was obtained (like SPYBALS, for example) it is possible to reduce this number. Additionally the task wavecal can be used to obtain new dispersion coefficients and zero-point differences.
* Archival instrument.
8 A detailed description can be found in the GHRS Instrument Handbook, version 6, May 1995. Available at http://www.stsci.edu/ftp/instrument_news/GHRS/topghrs.html. Back.
* Archival instrument.