HST Stellar Standards with 1% Accuracy in Absolute Flux
  Free of any atmospheric contamination, HST provides the best available spectrophotometry from the far-UV to the near-IR for stars as faint as V~16. The HST CALSPEC standard star network is based on 3 standard candles: the hot, pure hydrogen white dwarf (WD) stars G191B2B, GD153, and GD71, which have Hubeny NLTE model flux calculations that require the atomic physics for only one atom. These model flux distributions are normalized to the absolute flux for Vega of 3.46x10^{-9} erg cm^{-2} s^{-1} \AA^{-1} at 5556\AA using precise Landolt V band photometry and the V bandpass function corrected for atmospheric transmission by M. Cohen. The 3 primary WD standards provide absolute flux calibrations for FOS, STIS, and NICMOS spectrophotometry from these instruments on the HST. About 32 stellar spectral energy distributions (SEDs) have been constructed with a primary pedigree from the STIS data, which extends from 1150 \AA for the hot stars to a long wavelength limit of 1\mu m. NICMOS grism spectrophotometry provides an extension to 1.9\mu m in the IR for 17 of the HST standards and longward to 2.5\mu m for a few of the brighter stars. Included among these HST standards are Vega, the Sloan standard BD+17$^{\circ}$4708, 3 bright solar analog candidates, 3 cool stars of type M or later, and 5 hot WDs. In addition, 4 K giants and 4 main sequence A-stars have NICMOS spectrophotometry from 0.8-2.5\mu m. The WD fluxes are compared to their modeled SEDs and demonstrate an internal precision of 1-2%, while the A-stars agree with the Cohen IR fluxes to ~2%. Three solar analog candidate stars differ from the solar spectrum by up to 10% in the region of heavy line blanketing from 3000-4000 \AA and show differences in shape of ~5% in the IR around 1.8\mu m. 
Ralph C. Bohlin (Space Telescope Science Institute)