Absolute Flux Calibration for STIS First-Order, Low-Resolution Modes

Point source sensitivity curves are derived for the first order modes G140L, G230L, G230LB, G430L, and G750L, which collectively span a wavelength range from 1140Å to 10320Å. The curves are determined by comparing wide slit (52" x 2") observations of the spectrophotometric standard star GD153 to a pure hydrogen white dwarf model. The calibration is checked by comparing a calibrated STIS observation of the spectrophotometric standard star G191B2B to its pure hydrogen model. Except for G140L, there is no good evidence for changes in the sensitivity beyond ~1% through the end of 1997. Except for the G140L and the long wavelength part of G750L, the absolute flux calibration is accurate for observations in 1997 to ~3% for stars centered in the 52x2" slit. Preliminary results on the vignetting analysis for the first order modes suggest that the smooth changes in sensitivity as a function of distance along the slit are less than ±2 % for all first order modes, except for the extremities of G140L, where 5% differences in detector QE are observed. While the 52x2" slit is ideal for photometric continuum determinations, a narrower slit should be used for line profile measurements in order to minimize the contamination from the impure light in the wings of the PSF. For the narrowest slits, local inhomogeneities in the slit width cause a variable throughput along the slit (see the STIS Foibles page on the STIS Instrument WWW pages).