THE EFFECT OF EVENT SHAPE ON CENTROIDING IN PHOTON-COUNTING DETECTORS

High resolution, CCD readout, photon counting detectors employ simple centroiding algorithms for defining the spatial position of each detected event. The accuracy of centroiding is very dependent upon a number of parameters including the profile, energy and width of the intensified event. In this paper, we provide an analysis of how the characteristics of an intensified event change as the input count rate increases and the consequent effect on centroiding. The changes in these parameters are applied in particular to the MIC photon counting detector developed at UCL for ground and space based astronomical applications. This detector has a maximum format of 3072 x 2304 pixels permitting its use in the highest resolution applications. Individual events, at light level from 5 to 1000k events/s over the detector area, were analysed. It was found that both the asymmetry and width of event profiles were strongly dependent upon the energy of the intensified event. The variation in profile then affected the centroiding accuracy leading to loss of resolution. These inaccuracies have been quantified for two different 3 CCD pixel centroiding algorithms and one 2 pixel algorithm. The results show that a maximum error of less than 0.05 CCD pixel occurs with the 3 pixel algorithms and 0.1 CCD pixel for the 2 pixel algorithm. An improvement is proposed by utilising straight pore MCPs in the intensifier and a 70 mum air gap in front of the CCD.