We describe the design and development of a large format 40‐mm active diameter imaging photon detector. The design comprises an S20 semitransparent photocathode proximity focused onto a stack of two microchannel plates clamped back to back having channel length to diameter ratios of 80:1. Event readout is facilitated by a circular arc terminated resistive anode. The theoretical performance of the design is discussed with reference to gain measurements of the microchannel plates. Experimental results give an optimum position resolution of 100 &mgr;m which is spatially nonuniform and degrades towards the edge of the imaging field. We discuss the role of signal processing in this nonuniformity and describe design modifications which would achieve optimum imaging performance.