FAQ on FOC Photometry and Keywords

Q:  What is the absolute photometric calibration?

A:  The PHOTFLAM keyword in the header represents the constant
to multiply the observed photon count rate by to obtain F lambda.
This conversion constant is calculated assuming the source has a
constant F lambda over the filter bandpass, and then
calculating the expected count rate for the assumed DQE and
filter transmission curves (it is not derived directly from
observations of a standard star throught the same filter set as
is commonly done for ground-based observations). If you are
using wide band filters and your source deviates significantly
from a flat F lambda spectrum, blindly applying PHOTFLAM may
lead to significant errors. You are advised to use a tool such
as SYNPHOT or FOCSIM to relate the absolute flux to count rate
for the spectral shape, if availble, of your source. Beware of
redleaks!