Final Astro-2 calibration of the Hopkins Ultraviolet Telescope

We present the final calibration of the Hopkins Ultraviolet Telescope (HUT) for its flight during the Astro-2 space shuttle mission in 1995 March. Asp ects of mission operations and instrument performance that affect data qual ity are described in detail, as are the data reduction procedures applied t o the archived data. The sensitivity calibration is defined by a comparison between synthetic spectra and observations obtained in flight of the hot D A white dwarfs HZ 43, GD 71, GD 153, and G191-B2B; the synthetic spectra we re calculated by D. Koester using model parameters derived from fits to gro undbased spectra. The resulting flux-calibrated spectra for these stars dif fer from their respective model predictions by less than 3% at all waveleng ths, except at the cores of the Lyman lines where the observed line profile s are shallower than the predicted profiles. As an additional consistency c heck, the HUT spectrum of ED +75 degrees 325 was found to differ from the H ubble Space Telescope Faint Object Spectrograph spectrum by at most 5% in t he region of overlap. The wavelength scale and spectral resolution were cal ibrated by observations of symbiotic stars and of the coronal star alpha Au r. The spectral resolution was found to vary from 1.8 to roughly 4.5 Angstr om, depending on wavelength and on the instrument configuration. The intern al consistency of the HUT calibration provides a new and important test of white dwarf model atmospheres, as the predicted stellar spectra are more se nsitive to changes in model parameters or to shortcomings in the models the mselves at wavelengths shortward of 1100 Angstrom than at longer wavelength s. Combining this result with that of the Astro-l HUT calibration (that a s ynthetic spectrum of G191-B2B and laboratory flux standards gave results co nsistent within the laboratory measurement uncertainties) demonstrates that pure hydrogen white dwarfs with effective temperatures of 32,000-61,000 K may be used as primary flux standards down to the Galactic Lyman edge.