Optimizing the design of very high energy gamma ray telescopes

Abstract

Design strategies for gamma ray telescopes which utilize atmospheric Cherenkov emission to detect astronomical photons with energy ≲ few TeV are considered theoretically, e.g. threshold, field of view, coincidence techniques, filters, altitude of observing station and image selection. The design criterion used was to optimize the signal to noise ratio S √B , or figure of merit Q = S 2 B , where S is the number of photon triggers and B is the number of background events, including both cosmic rays and sky noise. Photon and hadronic showers were simulated using a large computer program, which samples from integral power law spectra with indices −1.5 and −1.25 (sources), and −1.65 (background). The analysis showed an optimum field of view of ∼1.5° and trigger threshold such that sky noise is ∼2–15% of total background. As this is high, a low noise threshold was also considered but hardly alters the conclusions. Fast coincidences between independent detectors provide a very efficient method of reducing the sky noise, e.g., improvements in Q of ∼4.2 and ∼7.5 for two- and threefold coincidences were obtained with spectral index −1.5. Fast timing to reject off-axis (nuclear) images is of limited use.