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GHRS Instrument Handbook

"Sensitivity" as a number is the conversion factor between measured counts per second per diode at some wavelength to ergs per square centimeter per second per Ångstrom. Those conversion factors have now been measured for all the gratings of the GHRS except G140M as part of instrument characterization (the measures for G140M were deferred to Cycle 5 because that grating was almost unused in Cycle 4). We have also confirmed that none of the measured sensitivities appear to change with time with one possible exception: There is preliminary evidence that the sensitivity of the GHRS is declining slightly for wavelengths below 1200 Å. This decline is no more than about 10% and there are possible systematic effects that have not been ruled out completely.

The Cycle 5 sensitivity monitoring program observes a standard star four times per year at a series of wavelengths for all of the first-order gratings of the GHRS and once per year for the echelle gratings. We are, of course, paying particular attention to the wavelengths below 1200 Å. Our experience with these observations allows us to note the following:

The stable performance of the GHRS means that it is impossible to improve substantially on the flux calibration by obtaining your own calibrations.