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:
- Repeated observations of the same star at the same wavelength (but at different times) should be repeatable to about 1%. This is the precision of sensitivity.
- Observations of the same star at different wavelengths are comparable to about 2 to 5%, depending on how close together the two wavelengths are. This value depends on our knowledge of the intrinsic fluxes of the calibration stars as well as knowledge of the GHRS and depends on such matters as continuum placement, for example.
- Fluxes on an absolute scale are good to within about 10%; this is the accuracy. This value of 10% is limited by systematic effects that are impossible for an observer to remove.
The stable performance of the GHRS means that it is impossible to improve substantially on the flux calibration by obtaining your own calibrations.