The Ultraviolet Calibration of the Hubble Space Telescope. I. Secondary Standards of Absolute Ultraviolet Flux and the Recalibration of IUE

Abstract

Spectral energy distributions of IUE standards are presented. The five UV standards, HD 60753, BD +75°325, HD 93521, BD +33°2642, and BD +28°4211, were well observed in the original calibration epoch of the IUE observatory and have been monitored for UV variability since the IUE launch in 1978. Only observations from the first year of IUE were used to define the standard fluxes. These five standard stars are the basis for evaluating later epoch IUE sensitivities and for calibrating the UV-sensitive instruments on the Hubble Space Telescope. An error in the original IUE calibration of Bohlin and Holm for the low-dispersion mode was discovered and corrected. The corrected IUE fluxes for BD +28°4211 and BD +33°2642 are compared to ground-based measurements of stellar flux below the Balmer limit by Stone. The corrected IUE absolute calibrations for the SWP and LWR cameras in the initial epoch are presented.