46.7 Photometry

Users should correct their fluxes for the decrease in sensitivity that may have occurred during the time between the observation and the decontamination done before the observation was taken.¹ (See the list of decontamination events in Table 46.1). The necessary correction can be estimated from Figure 46.3 or by using synphot (e.g., the calcphot task), which can now accommodate the time-dependent throughput via the CONTAM keyword which is specified as part of the OBSMODE (WF/PC-1/WFPC2 Instrument Science Report 96-02).

The absolute sensitivity calculation in WF/PC-1 is done assuming a flat spectrum (constant flux per wavelength); if the spectrum of the source is not flat, the STSDAS synphot package can be used to recalibrate the flux (see Chapter 3). Finally, corrections to the conversion factors (PHOTFLAM) may need to be applied if an alternate flatfield is used; use the photometry appropriate for the filter through which the observation was taken, regardless of the flatfield filter.