The future of imaging photon counting detectors for ground-based astronomy

Silicon Charge-Coupled Devices (CCDs) are the ubiquitous detector of choice for most ground based optical cameras given their high QE, low readout noise, good spatial resolution and large array size. However, they are integrating detectors, and as such have a limited temporal resolution determined by the readout frame rate. Imaging photon counters, on the other hand, can determine the location and arrival time of an individual detected photon which lends itself to studies of the intrinsic variability of astronomical sources. This topic is of fundamental importance, especially for the case of compact objects in stellar binary systems, stellar flares, and accretion disk phenomena. Most of these timing observations are currently performed by satellite-born X-ray instruments, but similar data can also be obtained from ground-based observatories at visible wavelengths using photon counting detectors. In this paper we review the recent and future improvements in the performance of imaging, photon counters, especially their optical QE, array size, spatial and temporal resolution and dark counting rates. We will compare them to conventional CCD devices and discuss the observational applications for which either or both can excel. We find that for certain applications (such as high time resolution observations, faint spectra and wavefront sensors for adaptive optics) imaging photon counting detectors can provide observations of superior signal-to-noise to CCD's.