On-orbit calibration of the Swift Ultraviolet/Optical Telescope(UVOT)

The Ultraviolet and Optical telescope (UVOT) is one of the three instruments on board of the SWIFT observatory. UVOT is on the cutting edge of our ability to observe and eventually help scientists to understand gamma-ray bursts. As any space-based telescope it requires both pre-flight and on-orbit calibrations. This paper is the first of a pair of papers presenting the initial on-board calibration of the UVOT. In particular, we'll discuss distortion, large and small scale sensitivity variations and the telescope point spread function.