The performance of a prototype photon counting imaging detector, being developed for the international UV space mission spectrum-UV, is presented. The detector is based on a 4-cm diameter, Z stack, high gain microchannel plate (MCP) intensifier endowed with a RbTe photocathode. The electron cascade generated by the MCP intensifier is transduced, via a phosphor screen and a 1:3.6 fiber optics reducer, into a 5 by 5 pixel2, quasi-Gaussian charge distribution on a 15 micrometer, 512 by 512 pixel2 format CCD matrix read out in the frame-transfer mode at 20 MHz, corresponding to 60 frame sec-1 in the full frame mode and to 220 frame sec-1 in the window (128 by 512 pixel2) mode. The data flow is acquired serially as to generate a 5 by 5 pixel2 event sash that sweeps dynamically the CCD matrix at the 50 ns place of the readout clock. Each and every event sash is searched for the presence of events whose charge distribution lie within set thresholds and satisfy given morphological rules, i.e. a peaked charge profile. The centroid coordinates of identified events are subsequently determined with sub-pixel accuracy and stored in an external, high resolution memory. The data acquisition and processing system, based on field programmable gate array technology, is well able to resolve the front MCP pore geometry (10 micrometer diameter pores at 12 micrometer pitch).