FINAL ASTRO-1 CALIBRATION OF THE HOPKINS ULTRAVIOLET TELESCOPE - A TEST OF WHITE-DWARF MODEL ATMOSPHERES AS FAR-ULTRAVIOLET CALIBRATION STANDARDS

We present the final calibration of the Hopkins Ultraviolet Telescope (HUT) for its flight during the Astro-1 space shuttle mission in 1990 December. The calibration is defined by a comparison between an observ ation obtained in flight of the hot DA white dwarf G191-B2B and the mo del atmosphere for this star calculated by P. Bergeron. We also presen t the postflight laboratory calibration of the HUT sensitivity, which we find to be in excellent agreement with the white dwarf calibration. The ratio of the white dwarf to the laboratory effective area calibra tion has a mean value of 1.0044 over the wavelength range 912-1840 Ang strom, a slope of 7.61 x 10(-6) Angstrom(-1) (which corresponds to a r elative difference of only 0.35% at either end of the spectrum), and f luctuations about the mean value with an rms amplitude of 3%. The unce rtainties in the laboratory calibration are 4% for the overall normali zation, 5% relative to the mean on wavelength scales of 50 to several hundred Angstrom, and an uncertainty in the overall slope that corresp onds to 1% at 1840 Angstrom and that increases smoothly to 3% at 912 A ngstrom. These results validate the use of white dwarf models as absol ute flux standards in the far-ultraviolet, especially for wavelengths shortward of Ly alpha, where previous experiments often disagreed by l arge factors. In particular, the normalization and wavelength dependen ce of the sensitivity calibration adopted by the Hubble Space Telescop e Faint Object Spectrograph is confirmed to within the uncertainties o f our laboratory calibration, while the normalization of the IUE final calibration is found to be low by an amount just exceeding our 1 sigm a uncertainty. Fits to the spectra of both G191-B2B and the hot DA whi te dwarf HZ 43 yield effective temperatures and surface gravities in g ood agreement with values derived from Balmer line profiles. We also p resent a comparison with Voyager spectra for the stars G191-B2B, HZ 43 ED +28 degrees 4211, pi Aqr, and HD 25443. When averaged over broad b ands (912-1050 Angstrom, 1050-1180 Angstrom), fluxes from the two inst ruments typically agree to within 10%.