1. HST Calibration Prior to launch a concerted campaign to establish standards was pursued on ground-based telescopes and from UV archives Turnshek et al. 1990. “An atlas of HST photometric, spectrophotometric and polarimetric calibration objects” Software repository of calibration data – Calibration Data Base System (CDBS) holding spectra of calibration targets and much more Most of development in area of spectrophotometric standards, headed by Ralph Bohlin who was and is Project Lead, using FOS and then STIS ESO Faculty Meeting 2004 October 29 J. R. Walsh

2. HST Calibration – UV-optical spectrophotometric standards Four WD stars adopted as ‘primary’ HST standards – G1919B2B, Hz43, GD153 and GD71 IUE and ground spectrophotometry (Oke, 1990) joined. Photometry used for refining absolute fluxes Initially Faint Object Spectrograph (FOS) used to refine UV fluxes. Then STIS for UV and optical. Match of observed fluxes to NLTE WD model atmospheres (Finlay, Hubeny) Set of ‘secondary’ HST spectrophotometric standards established. WD fluxes extended to near-IR by models. ESO Faculty Meeting 2004 October 29 J. R. Walsh

3. HST Calibration – applications HST WD primary standards to refine Vega fluxes! HST standards used to calibrate instrument throughputs All component throughputs measured and or modelled on ground →CDBS and accessed by stsdas.synphot and ETC’s On-orbit monitoring to refine throughput tables (filter aging, polymerization etc) ESO Faculty Meeting 2004 October 29 J. R. Walsh