16514

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