Further calibration of the Swift ultraviolet/optical telescope

Abstract

The Ultraviolet/Optical Telescope (UVOT) is one of three instruments onboard the Swift observatory.The photometric calibration has been published, and this paper follows up withdetails on other aspects of the calibration including a measurement of the point spreadfunction with an assessment of the orbital variation and the effect on photometry. A correctionfor large-scale variations in sensitivity over the field of view is described, as well asa model of the coincidence loss which is used to assess the coincidence correction in extendedregions. We have provided a correction for the detector distortion and measured theresulting internal astrometric accuracy of the UVOT, also giving the absolute accuracy withrespect to the International Celestial Reference System. We have compiled statistics on thebackground count rates, and discuss the sources of the background, including instrumentalscattered light. In each case, we describe any impact on UVOT measurements, whether anycorrection is applied in the standard pipeline data processing or whether further steps arerecommended.