Absolute flux distribution of the SDSS standard BD+17°4708

Secondary flux standards are established by measuring their brightness relative to primary standard stars. The Hubble Space Telescope (HST) primary standards are the three pure-hydrogen white dwarf (WD) flux standards that determine the sensitivity calibration for the Space Telescope Imaging Spectrograph. STIS observations have defined the flux of the Sloan Digital Sky Survey (SDSS) standard BD +17degrees4708 from 0.17 to 1.0 mum with an uncertainty of less than 0.5% relative to the HST primary standards, as verified by two independent sets of photometry. The two CCD cameras, HRC and WFC, in the ACS instrument package on HST confirm the average ratio of BD +17degrees4708 to the three fainter WD primary standards to better than 0.5% if a few of the ACS filter bandpasses have small wavelength shifts. With an uncertainty of less than 0.5% in the transfer of the flux calibration from the primary standards, the accuracy of the BD +17degrees4708 relative flux distribution is essentially the same as that of the primary WDs, that is, similar to2% in the relative fluxes over the 0.17-1 mum range. Our flux distribution agrees with the SDSS fluxes to within their quoted 3% uncertainty.