GHRS Instrument Handbook
This version of the GHRS Instrument Handbook has had significant rewriting, mostly to incorporate information about the proposal writing software called RPS2. We have also added a chapter on calibrations for the GHRS, as well as a few updates since version 5.0 that were issued electronically. Please bring errors to the attention of a GHRS Instrument Scientist (see back of title page).
We anticipate that Cycle 6 will be the last for the GHRS. This has three important implications for proposers:
The second servicing mission for HST is scheduled to take place in February, 1997. Since Cycle 6 starts in July, 1996, there is only about half a cycle in which to complete Cycle 6 GHRS observations. Even if time is awarded, GHRS observations that are not completed before the servicing mission will not be carried forward into a later cycle. Time-critical observations that must take place after January, 1997, and are intended to be done with the GHRS will be executed only if a launch delay makes that possible. The shortness of Cycle 6 for the GHRS also places other constraints on scheduling and makes use of early acquisitions or complex linkages between observations especially difficult.
Because Cycle 6 is short for both the GHRS and FOS, it is STScI's intention to schedule spectroscopic observations early in Cycle 6 to the greatest extent possible so that as many FOS and GHRS programs may be completed as is possible.
The Space Telescope Imaging Spectrograph (STIS) will replace most of what the GHRS can do (consult the STIS Mini-Handbook for details). However, the GHRS has a few unique capabilities that STIS will not duplicate. One of those is a demonstrated capability to obtain very high signal-to-noise (1,000 or more) spectra. STIS, by comparison, is expected to be able to provide up to about 100. The GHRS can also count very rapidly: rates of 20,000 counts per second per diode are correctable to a linearity of 1%. STIS, on the other hand, will be limited to a local count rate of ~50 per second per pixel. For most purposes STIS should offer a substantial improvement over the GHRS, but proposers should be aware of these unique qualities of the GHRS in preparing for Cycle 6.
We remind you that the STIS Mini-Handbook is available to outline the capabilities that will be available to you in Cycle 7. You may wish to take STIS into account in making long-term observing plans. In particular, the STIS Mini-Handbook provides a comparison of the sensitivities of STIS and the GHRS for modes providing comparable quality spectra.
In some cases the G140L grating on Side 1 may provide an efficient means of obtaining a low-resolution spectrum of a source, but acquiring that object can be difficult or impossible with the GHRS Side 1 because of the limited response of mirror N1 and the maximum permissible STEP-TIME of 12.75 seconds. There are two ways to overcome this problem:
- Acquire the object with Side 2 of the GHRS (mirror N2), then observe with Side 1. Using this technique will add about 40 minutes of overhead time involved in switching Sides, but often that can occur when the spacecraft is in occultation anyway.
- Acquire the object with the Faint Object Spectrograph and then move it to the LSA of the GHRS. The positions of the COSTAR mirrors for the FOS and GHRS are quite close, so that the movement of an object from an FOS blue side aperture to the LSA of the GHRS is only about 1 arcmin, a small enough motion to ensure that the object will fall within the LSA because one set of guide stars will suffice. This method should not be used for SSA observations, nor will it usually work for objects acquired with the red side of the FOS.
A special commanding option called FLYLIM can be invoked to reject noise in the GHRS when the object observed is significantly fainter than the level of the background noise. Although only applicable in special situations, it can be very effective. However, using FLYLIM also involves some real risks that can be avoided by other methods. Please see Section on page 118.
The sensitivity values listed in Chapter 8 are those measured after the installation of the COSTAR mirrors and they are therefore up to date. Version 5 of this Handbook had interim values of the sensitivity.
Cycle 6 and the End of the GHRS
A Short Cycle 6
Loss of GHRS Unique Capabilities
Replacing the GHRS
Acquiring Faint Objects with the GHRS or FOS
Noise Rejection for Very Faint Objects