July 1995
Abstract
The cycle 4 absolute photometric calibration of the Faint Object Spectrograph is derived from observations of eight spectrophotometric standard stars, including four white dwarf (WD) stars with pure hydrogen atmospheres, in the 4.3" square and 1" circular entrance apertures for 14 detector-disperser combinations (six for the blue digicon, and eight for the red). Except for red H19 and red L15, the observations of the eight stars over the period of February 1994 through July 1995 show no systematic variation in the sensitivity with time.
During the period from February 1994 through July 1994, a ~2 percent drop in sensitivity for the red H19 mode is evident. However, from the period of July 1994 through July 1995, the red H19 sensitivity increases by 3 to 8 percent. The red L15 behaves similarly below 2200Å.
Changes in the FOS flux calibrations relative to those in use by the pipeline since 1994 March 21 are typically less than ~3% in high dispersion for the 4.3" A-1 and the 1" B-3 apertures and exceed 10% only in small regions for the low dispersion modes. Small aperture fluxes differ by over 10% shortward of 1300Å for the 0.3" B-2 aperture, since the transmissions of the small apertures were not measured until after 1994 March.
The residuals of the FOS fluxes in comparison to the WD pure hydrogen model atmospheres are less than ~2% in high-dispersion and provide the best measure of small scale errors in the FOS fluxes. The overall uncertainty of the FOS flux calibration for point sources is estimated to be 3%.
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