We report on photon-counting data taken with a laboratory prototype detector for the ASTROSAT UV imaging telescope, and the selection of the centroiding algorithms and parameter settings for flight. Thresholds are discussed for event recognition. The optimum event centroid algorithm is a 5 × 5 pixel weighted mean, after bias removal accurate to 2%-3%. This can be achieved within each readout frame by using the lowest value corner pixel of the photon event subarray. We also discuss other corner pixel thresholds for flagging double-photon events, and methods of correcting centroiding errors with different algorithms and event rates.