A calibration of the ROSAT HRI UV leak

The purpose of this paper is to present a detailed investigation of the Ult raviolet and visible sensitivity of the high-resolution imager (HRI) onboar d the ROSAT X-ray satellite. We provide observational evidence that a recen tly published model (Zombeck et al. 1997) of the out-of-band quantum effici ency of the HRI overpredicts the detector response longward of approximatel y 4000 Angstrom. The contamination of the HRI is limited to the UV bandpass below approximately 4000 Angstrom. Based on the optical properties of our target stars, our ROSAT HRI and PSPC observations, and UV fluxes measured w ith the TD1 satellite, we provide an accurate calibration for the UV leak f lux and present formulae to estimate UV leak count rates based on UV (1965 Angstrom) fluxes or stellar U magnitudes.