ISR 2022-07: Update of the STIS CTE Correction Formula for Stellar Spectra
October 25, 2022R. C. Bohlin and S. Lockwood
The correction formula for Charge Transfer Efficiency (CTE) that is used in the HSTCAL CALSTIS pipeline CCD data reduction has not been significantly updated since 2006. Correcting for CTE losses is crucial to the goal of 1% precision in the STIS spectrophotometric fluxes that are the basis for all HST and JWST flux calibrations. Precision in the CTE correction is especially relevant for the faintest flux standards, where the amount of correction can exceed 20%. The comparison of new datasets of STIS spectra and ACS photometry of faint stars reveals the required updates to the parameters of the STIS CTE correction formula. After replacing the original with the new parameters, the change in the spectral energy distribution (SED), i.e. the flux decrease in physical units, for a very faint star NGC2506-G31 ranges from 4 to 6% over most of the G430L and G750L spectral wavelength ranges. No observations of the faint stars with G230LB or the medium dispersion modes were made; but the CTE correction depends only on signal level and should apply to all CCD spectroscopic modes. With the new formulation for the STIS CTE correction the STIS, ACS, and WFC3 flux measures are in accord at the 1% level, not only for the primary standards and other stars in the neighborhood of V=13 and brighter, but also now for fainter stars in the V=16 range.