Synphot Data User's Guide
2.7 Background Calculation (WFC3)
bkgkeyword is used to perform throughput and emission calculations pertaining to the background signal component of grism observations.
In grism observations, a given detector pixel will receive source signal from only a small wavelength interval of the dispersed source spectrum, but it will receive background signal from the entire bandpass of the grism. A special throughput table must therefore be used to correctly compute the detected signal from a background spectrum, which gives the transmission of the grism over its entire bandpass.
The bkg keyword is only valid when grisms are specified as part of the obsmode.
We expect to implement this functionality for grism modes in other HST instruments in the future.
Space Telescope Science Institute
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