DOI: 10.57780/esa-c1yis2q
title: 16514

description: The CALSPEC archive contains the SEDs of the standard stars that will be used for the absolute flux calibrations of JWST comma  Roman comma  and ground-based telescopes. The CALSPEC SEDs are based mostly on STIS  and the ACS flux calibration agrees with STIS by definition comma  because the same three relatively bright standard stars are used for both calibrations. However comma  properly cross-calibrated for bright stars does NOT mean proper cross-calibrated for faint stars comma  because the two instruments have different non-linear corrections for CTE losses. While the provenance of the three bright primary standards (G191B2B comma  GD153 comma   and  GD71 with V=11.8-13.3) and brighter stars is robust comma  the precision of fainter stars depends on substantial corrections for CTE losses. These fainter stars will be the primary standards for COS comma  JWST comma  Roman comma  and many modes of modern and larger telescopes. Differences between STIS and ACS fluxes for faint standards at the current epoch will provide vital measures of the relative CTE correction errors between the two HST instruments and will quantify any required updates to their relative CTE correction algorithms. This cross-calibration is especially germane in 2021 comma  because CTE losses increase with time making the current epoch the most sensitive to CTE corection errors and providing the best leverage for improving the correction algorithms.  Four Faint Standards comma  WD1057+719 comma  WD1657+343 comma  WD0308-565 comma  and SDSSJ151421 (V=14-16.5) comma  are targeted for upgrading their S-N and for quantifying their repeatability in two 12 orbit STIS programs (16028  and  16437) in cycles 27-28. The precision of these flux distributions in the STIS G430L and G750L CCD modes is limited by the applicability of the old empirical correction algorithm of Goudfrooij et al. (2006) comma  where the time dependence is now extrapolated virgul15 years past the date of derivation. Any improvement in the STIS CTE correction directly affects the extrapolation to longer wavelengths for these four JWST standards and for any other faint JWST or Roman standard.  In particular comma  SDSSJ151421 shows indications of a virgul2% discrepancy with respect to WFC3 photometry (Bohlin et al. 2020 comma  AJ comma  160 comma  21 comma  Fig. 14). The amount of its CTE correction ranges from 7-20% over 5500-10000A. Thus comma  even the estimated uncertainty of 20% of the correction exceeds 2% at wavelengths longer than 8000A comma  where a 10% CTE loss is predicted.  Therefore comma  to achieve the challenging goal of 1% accuracy for faint standards comma  the antique STIS CTE correction formula must be verified or upgraded comma  while some questions about the ACS CTE correction have been raised recently (Chiaberge comma  private comm. comma  Bohlin comma  Ryon comma   and  Anderson ACS ISR 2020-08). An excellent test is via an ACS-STIS cross-calibration. ACS provides a better STIS cross-calibration than WFC3 because of the use of a single reference point near pixel 3585 comma 3585 comma  where all the ACS calibration observations of the primary standards are concentrated. The ACS-STIS CTE corrections must be adjusted to correct any ACS-STIS mismatch