GHRS Instrument Handbook
This mode is sometimes referred to as direct downlink. A normal ACCUM exposure is the best way to get a good spectrum because all the features of the spectrograph are available to you: automatic compensation for the motion of the spacecraft along the line-of-sight, rejection of high-noise subspectra with CENSOR, use of FP-SPLIT, COMB, and STEP-PATT to optimize data quality, and so on. However, there are times when ACCUM cannot obtain successive spectra as quickly as is needed to probe a particular phenomenon.
In those cases you can use RAPID mode. The data are read from the detector at the end of each short integration, either to the science tape recorder on HST or through TDRSS to the ground. Data obtained in RAPID mode require special handling by the observer to correct for some of the effects (especially doppler shifts) that are automatically compensated for in ACCUM mode.
As for an ACCUM, you should specify the science aperture and the spectral element. You may also choose to observe WAVE as target to get a wavelength calibration. The only other parameter you may specify is SAMPLE-TIME, which is the length of each separate exposure that is read to the ground. The default SAMPLE-TIME is also the minimum, 0.05 seconds. SAMPLE-TIME may be incremented in 0.05 second values up to a maximum of 12.75 seconds. Use of a very short SAMPLE-TIME and/or use of RAPID mode for extended periods can cause scheduling problems because of the very high data volumes that are generated. In particular, a SAMPLE-TIME of less than 0.33 sec records data at the 1 Mb rate and so can proceed for no more than about 20 minutes before filling the onboard tape recorder. A SAMPLE-TIME of 0.33 seconds or more results in a data rate that can be sustained for about 10 hours.