GHRS Instrument Handbook
Instrument Summary -- Why Use the GHRS?
Because of the difficulties of working with and communicating with a satellite in low-earth orbit, and in order to make HST more efficient, virtually all actions taken by the spacecraft are planned weeks in advance. Only a small fraction of HST's time can be used for real-time actions that are at the discretion of the observer, and even then the only possible action is deciding which object in the field should be centered in the aperture before a subsequent observation is begun.
This need for detailed planning of HST observations lies at the heart of the apparent complexity of the use of the spacecraft and its instruments. At the same time, by carefully laying out every aspect of what you want done you will find yourself with a better understanding of what actually happens and more confidence that the desired results will be achieved.
All HST observations begin with an acquisition. An acquisition can be as simple as blindly pointing to particular celestial coordinates, although such a procedure is unlikely to succeed with the GHRS because its entrance apertures are small. For the GHRS, an acquisition usually means a pointing to precisely specified coordinates, then making small motions of the telescope in a spiral pattern to sample the region of sky in the vicinity of the coordinates, and then a peakup motion to center a star in the aperture after on-board software has determined its location. Variations include offsetting from the acquired star to another nearby object or moving the star to the small aperture. In rare cases it may be necessary to perform an interactive acquisition, in which the observer specifies the object in real time. An intermediate possibility is to take an image with one of HST's cameras (or with the GHRS itself) in advance of the spectroscopic observation (by one to two months) and to then derive precise coordinates from that image (an early acquisition). For very faint objects, especially those to be observed with Side 1 of the GHRS, it is possible to acquire the object with the Faint Object Spectrograph before moving it to the LSA.
Once the star has been properly positioned in the appropriate aperture of the GHRS, science observations may begin. In some cases you may wish to use IMAGE mode, which can map the LSA at ultraviolet wavelengths, but in general this part means dispersing the light with one of the gratings and adding up the counts to form a spectrum. A RAPID mode also exists to record spectra that change on very short time scales. The GHRS has no independent microprocessor and so depends on the spacecraft's computer and memory for control of its operations. One implication of that dependence is that there is a maximum number of commands that can be stored at any one time. Since those commands are generally loaded into the spacecraft only a few times per day, that limitation restricts the total number of GHRS exposures that may be made in a 24 hour period (see ``Memory Usage'' on page 52). At the same time, image motion within the instrument that is induced by the earth's magnetic field is best dealt with by making individual exposures no longer than about 5 or 10 minutes, thereby increasing the total number of exposures you need to make to get a science observation. In some cases these requirements come into conflict and compromises must be made to accommodate science goals.
Some other relevant aspects of scheduling HST observations are:
- Objects in most regions of the sky "rise" and "set" and will be available for science observations for about half of an orbit (about 50 minutes). Longer exposures get spread over several orbits, with a reacquisition of the guide stars at the beginning of each orbit. Some objects sometimes fall within HST's Continuous Viewing Zones (CVZs), which enables them to be observed for long times at high efficiency; see the Call for Proposals for details on taking advantage of the CVZs.
- The orbit of HST passes through the South Atlantic Anomaly (SAA), which is a region in which the background count rate is very high. The scheduling software simply stops the counting of photons during times when the spacecraft is within the SAA.
- GHRS observations are interrupted if HST goes behind the Earth while the observation is in progress. This is called Earth occultation.