STIS Flux Calibration

The low dispersion STIS spectrophotometric flux calibration must account for all instrumental non-linearities and all changes in sensitivity with time and temperature. An empirical algorithm for CCD Charge Transfer Efficiency (CTE) is presented along with the wavelength dependent sensitivity changes. Precise STIS spectrophotometry includes corrections for CTE losses as large as 20% for faint signals and low sky, for MAMA non-linearities of ∼ 2%, and for loss in system throughput that currently reaches a maximum ∼ 15% at 1625 Å with an average loss of more than 10% for G140L spectra on the FUV-MAMA. Newly available LTE and NLTE models from I. Hubeny’s TLUSTY code are compared to the old flux distributions for the Koester/Finley LTE models of the four primary flux standards. The uncertainty in the corrections and in the flux standards are all small enough, so that flux distributions relative to 5500 Å in the V -band can be measured in the photometric 52×2 arcsec slit to a precision of ∼ 1% over much of the 1150–10,000 Å STIS wavelength coverage. The NLTE model flux distributions are probably correct within ∼1% to 2.5 μ in the IR; and a model for the total uncertainty is presented.