1999A&A...342L..17B

The purpose of this paper is to present a detailed investigation of the Ultraviolet and visible sensitivity of the high-resolution imager (HRI) onboard the ROSAT X-ray satellite. We provide observational evidence that a recently published model (Zombeck et al., 1997ApJ...487L..69Z) of the out-of-band quantum efficiency of the HRI overpredicts the detector response longward of approximately 4000Å. The contamination of the HRI is limited to the UV bandpass below approximately 4000Å. Based on the optical properties of our target stars, our ROSAT HRI and PSPC observations, and UV fluxes measured with the TD1 satellite, we provide an accurate calibration for the UV leak flux and present formulae to estimate UV leak count rates based on UV (1965Å) fluxes or stellar U magnitudes.