Development of a high-time-resolution optical photon-counting instrument IMONY for fast transient astronomy

Abstract

IMONY is a high-time-resolution optical photon-counting instrument developed for fast transient astronomy. This system uses a 64-pixel Geiger-mode APD (also known as SPAD) array and FPGA-based readout electronics to record the detector channel and arrival time of each photon with 100-ns resolution. Earlier versions of IMONY were used for optical observations of the Crab pulsar and for coordinated optical–radio campaigns. To improve the compactness and deployability of the instrument, we developed a new front-end board based on the FGATI analog ASIC and integrated it with the detector, FPGA readout, timing system, power distribution, and mechanical structure. The upgraded system was installed on the Seimei telescope and operated for observations. Its on-sky count-rate response was evaluated using stars observed with several optical filters and was found to be approximately linear over the tested range. This compact FGATI-based detector unit was also applied to a portable configuration for high-time-resolution stellar-occultation observations.