GHRS Instrument Handbook
Instrument Summary -- Why Use the GHRS?
In preparing a Cycle 6 GHRS proposal, bear in mind that STIS will soon be available. STIS will be able to do most of what the GHRS can do, only more quickly. Thus there are observations which might best be done with STIS. However, the GHRS also has some capabilities that STIS will not provide, namely the means to obtain spectra of very high signal-to-noise and an ability to count photons at very high rates.
The best achievable signal-to-noise with STIS is likely to be about 100 because of inherent limitations in the stability of the flat fields of the MAMA detectors. On the other hand, the GHRS has been shown to be able to obtain signal-to-noise ratios in excess of 1,000 when special techniques are employed. We suggest that you consult Gilliland et al. (1992) and Lambert et al. (1994) to see how the GHRS may be used in this way.
For bright objects, STIS will be limited to a maximum local count rate of approximately 50 counts per second per pixel and a global count rate of 300,000 counts per second. However, the GHRS, even at a rate of 20,000 counts per second per diode, can be corrected for paired pulses to an accuracy of about 1%. Thus there are very bright objects for which excellent spectra can be obtained with the GHRS in a short time, whereas STIS may need substantially longer because a neutral density filter would be required. We suggest that you consult the STIS Mini-Handbook to determine how well STIS is likely to perform, and to emphasize the better efficiency of the GHRS in your proposal if there is a substantial difference in exposure time.