An Algorithm for Correcting CTE Loss in Spectrophotometry of Point Sources with the STIS CCD

The correction for the change in sensitivity with time for the STIS CCD modes is complicated by the gradual loss of charge transfer efficiency (CTE) of the CCD. The amount of this CTE loss depends on time in orbit, the location on the CCD chip with respect to the readout amplifier, the stellar signal strength, and the background level. Primary constraints on our correction algorithm are provided by measurements of the CTE loss rates for simulated spectra (tungsten lamp images taken through slits oriented along the dispersion axis) combined with estimates of CTE losses for actual stellar spectra in the first order CCD modes. The main complication is the quantification of the roll-off of the CTE losses for weak stellar signals on non-zero backgrounds. This roll-off term is determined by relatively short exposures of primary standard stars along with the G750L series of properly exposed AGK+81D266 monitoring data, where the observed changes in response over time are primarily CTE losses and not sensitivity degradations. After accounting for CTE losses and after an iterative determination of the optical system throughput losses, the CTE correction algorithm is verified by comparing G230L MAMA fluxes of faint standard stars with G430L fluxes in the overlap region around 3000Å. For spectra at the standard reference position at the CCD center, CTE losses as big as 20% are corrected to within 1% at high signal levels and with a precision of ~2% at ~100 electrons after application of the algorithm presented here. Copyright© 2003 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. STIS Instrument Science Report 2003-03R