Paper II: Calibration of the Swift ultraviolet/optical telescope

TL;DR

This paper details the calibration procedures for the Swift UVOT, including point spread function measurement, sensitivity correction, distortion correction, and background analysis, to improve photometric and astrometric accuracy.

Contribution

It provides comprehensive calibration updates for UVOT, enhancing data accuracy through new measurements and correction models not previously published.

Findings

Measured the point spread function and orbital variation effects.

Developed a correction model for sensitivity variations across the field.

Assessed background sources and their impact on measurements.

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 with details on other aspects of the calibration including a measurement of the point spread function with an assessment of the orbital variation and the effect on photometry. A correction for large scale variations in sensitivity over the field of view is described, as well as a model of the coincidence loss which is used to assess the coincidence correction in extended regions. We have provided a correction for the detector distortion and measured the resulting internal astrometric accuracy of the UVOT, also giving the absolute accuracy with respect to the International Celestial Reference System. We have compiled statistics on the background count rates, and discuss the sources of the background, including instrumental scattered light. In each case we describe any impact on UVOT measurements, whether any correction is applied in the standard pipeline data processing or whether further steps are recommended.