Absolute flux calibrations have been derived for the low-dispersion modes of the IUE cameras through observations of ultraviolet photometric standard stars, as well as observations and models of the white dwarf star, G191-B2B. The wavelength-dependence of the inverse sensitivity function (S) for each camera has been determined by comparison of IUE observations of the white dwarf star with model atmosphere calculations that were provided to the IUE project by D. Finley. The overall zeropoint of the calibration curves has been set by applying the white dwarf derived S values to IUE observations of ultraviolet photometric standard stars and comparing these results with OAO-2 measurements in the 2100-2300Å band (see González-Riestra, Cassatella, and de la Fuente 1992 for details regarding the calibration procedures).
The final S curves for the LWP, LWR (ITFs A and B), and SWP, defined in 15Å bins for the long-wavelength cameras and 10Å bins for the SWP and fit with spline curves, are listed in Tables 11.1-11.4. The absolute flux at a given wavelength, F (ergs sec-1 cm-2 Å-1), is computed as follows:
where is the extracted net flux in FN units, S is the inverse sensitivity value at that wavelength, and teff is the effective exposure time in seconds. The inverse sensitivity value at a particular wavelength is determined by quadratic interpolation of the tabulated values for a given camera.
The effective exposure time for non-trailed (e.g., point, extended, flat-field) sources is derived from the original commanded exposure time, tcom, and takes into account the effects of On-Board Computer (OBC) tick rounding and the camera rise time, trise, as follows:
where the values of trise for each camera are taken from González-Riestra (1991) and are 0.123, 0.126, and 0.130 for the LWP, LWR, and SWP cameras, respectively. Tick rounding results from the integer arithmetic used by the OBC in commanding exposures. Effective exposure times for large-aperture trailed observations are determined according to:
where Trail length is the trail path length of the aperture in arcsec, Trail rate is the effective trail rate in arcsec/sec, and Passes is the number of passes across the aperture. Because the OBC uses integer arithmetic in calculating fixed rate slews, there is a truncation in the commanded trail rate. This ``rounding off'' is similar to the OBC quantitization of non-trailed exposure times. The effective trail rate is calculated using the following equation:
where LSB is the least significant bit (0.03001548/32 arcsec/sec) and TRcom is the commanded trail rate.