S-Cam: A cryogenic camera for optical astronomy based on superconducting tunnel junctions

S-Cam is a cryogenic camera for ground-based astronomy using superconductin g tunnel junctions (STJ's). It has been designed as a technology demonstrat or, aiming to prove the potential of a new generation of single photon-coun ting detectors at a ground-based telescope. The camera is based on a 6 x 6 array of Ta-Al Josephson junctions, operating at about 350 mK and individua lly read nut. Fur each detected photon, the absorption position, the arriva l time, and the corresponding energy are measured, In this paper, we provid e an overview of the cryogenic detector performance, a description of the S -Cam system, and a summary of the results obtained both during testing at E STEC and during actual observations at the William Herschel Telescope (WHT) in La Palma, Spain. Initial astronomical observations were performed on th e Crab pulsar, a neutron star spinning with a period of 33 msec at about 60 00 light years from Earth.