Hubble Space Telescope Flux Calibration. I. STIS and CALSPEC

The precision of absolute flux measurements is quantified for the Space Telescope Imaging Spectrograph (STIS), which is the most important contributor to the CALSPEC standards. With our goal of 1% absolute accuracy, the documentation of the Hubble Space Telescope/CALSPEC flux scale and its subpercent repeatability are of prime importance. While the 1? rms broadband repeatability for G140L is 0.7%, the other four STIS spectral modes show excellent photometric repeatability of 1?& xfffd;=& xfffd;0.2%. The custom data processing that is required to achieve these results is discussed in detail. In particular, the correction for the wide extraction heights required for the saturated Vega and Sirius data is found to vary with time, which results in revised spectral energy distributions that decrease by as much as 2.5% for Sirius and 2% for Vega at 1 & xfffd;?m.