GHRS Instrument Handbook
Reference Information on Instrument Performance
This table shows how each STEP-PATT pattern measures the background for a science exposure. Listed are the STEP-PATT number, the number of spectrum bins for which substepping occurs, the number of background bins measured, the diodes used to measure the background, the fraction of the total time spent measuring flux on the science diodes, the gratings for which the STEP-PATT is appropriate, and the shortest exposure that can be used for that pattern. The shortest exposure has been computed assuming COMB=4, FP-SPLIT=NO, and STEP-TIME=0.2. These minimum exposures can be reduced by half if COMB=2 is used and by a factor of four for COMB=NO. The complete expression for the minimum exposure time is:
where Nbackground is the number of background bins (col. 3), Nspectrum is the number of spectrum bins (col. 2), and ratio is from column 4. The minimum STEP-TIME is 0.2 sec.
You should specify your exposure time as an integral multiple of tmin.