The RANICON for Ground Based Optical Astronomy

We have developed a versatile 2D photon counting imaging system that determines the location and arrival time of a detected photon with high precision and reliability. It makes use of a standard proximity focused semi-transparent photocathode to convert photons to electrons, coupled to a cascaded stack of microchannel plates acting as the position sensitive electron signal amplifier. A continuous uniform resistive sheet with appropriately shaped electrodes provides the means for encoding the simultaneous location and arrival time of each detected event with minimal spatial distortion and time delay. The four anode signals resulting from the charge redistribution on the plate are amplified and fed into a high speed signal processing electronics system that produces 12 bit x and y addresses for each event. A data acquisition system based on an IBM PC-AT builds up a 1024x1024 image from this asynchronous stream of x, y values that can be archived to hard disk or to a high speed, high volume tape. Laboratory measurements indicate that this Resistive Anode Image Converter (RANICON) system is presently capable of locating an event anywhere on the surface of a 25 mm diameter active area with a time resolution of ~ 1 microsecond and a spatial precision of 45 microns FWHM corresponding to 1000x1000 standard 25 micron pixels at 6000A. The data acquisition system can presently collect data at rates up to 50,000 events per second without measurable performance dearadation while the detector is linear to at least 15 counts s−1pixel−1for point sources and 1 MHz for full field illumination for an S-20 photocathode refrigerated to -20°C and exhibiting a peak quantum efficiency of ~20% at 4000A. Image distortion anywhere in the field is found to be negligible and the overall response of one device uniform and stable over ~ year of operation. Recent results of tests of the system on the McGraw-Hill 2.4m telescope at Kitt Peak are presented to illustrate its capabilities in the field.