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 detec ted event. The accuracy of centroiding is very dependent upon a number of parameters including the profile, energy and width of the intensif ied event. In this paper, we provide an analysis of how the characteri stics of an intensified event change as the input count rate increases and the consequent effect on centroiding. The changes in these parame ters are applied in particular to the MIC photon counting detector dev eloped at UCL for ground and space based astronomical applications. Th is detector has a maximum format of 3072 x 2304 pixels permitting its use in the highest resolution applications. Individual events, at ligh t 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 wer e strongly dependent upon the energy of the intensified event. The var iation in profile then affected the centroiding accuracy leading to lo ss of resolution. These inaccuracies have been quantified for two diff erent 3 CCD pixel centroiding algorithms and one 2 pixel algorithm. Th e 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 algorit hm. An improvement is proposed by utilising straight pore MCPs in the intensifier and a 70 mum air gap in front of the CCD.