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!
