Compute the scale factor for converting count rate to flux density. This
requires values for the keywords GRAPHTAB, COMPTAB, FILTNAM1, FILTNAM2, and for
f/96 also FILTNAM3 and FILTNAM4, as described below for UNICORR. Values are
computed for the inverse sensitivity, zero point, pivot wavelength, and RMS
bandwidth; these are saved in the keywords PHOTFLAM, PHOTZPT, PHOTPLAM, and
PHOTBW respectively. Multiplying pixel values by the value of PHOTFLAM and
dividing by the exposure time (EXPTIME) converts from counts to Flambda, flux
per unit wavelength. That is, Flambda = counts * PHOTFLAM / EXPTIME (ergs /
Angstrom / sec/ cm**2). The ST magnitude may then be obtained from STmag = -2.5
* log (Flambda) + PHOTZPT.
