S-Cam 2 was the second prototype of a cryogenic camera for ground-based astronomy, based around a 6×6 array of Ta-Al STJ's. S-Cam 2 was an improved version of S-Cam 1, which was tested on the William Herschel Telescope in La Palma (Canary Islands) in early 1999. S-Cam 2 was used on three successful observing campaigns on the same telescope in December 1999, April 2000 and September/October 2000. Work is now advanced on S-Cam 3, a completely new instrument incorporating many technological improvements, including a larger array with 120 pixels. It is intended that S-Cam 3 will see first light in the latter quarter of 2003. Most of the technical information on these pages currently relates to S-Cam 2.
The STJ Detector Array
Ta-Al STJ's have high responsivities, high detection efficiencies (~70% in the visible), adequate resolving power (~10 at 300 nm) and very fast response times (on the order of 10 microseconds). After testing different configurations, the array chosen for S-Cam 2 consisted of 36 diamond-shaped 25 micron devices, with a spacing between pixels of 4 microns. The detectors are fabricated from an original Ta-Al-AlOx-Al-Ta multilayer, deposited by sputtering onto a highly polished 520 micron-thick sapphire substrate. The thick aluminium layer gives S-Cam 2 a higher responsivity than its predecessor, which - coupled with improved IR rejection - resulted in an enhanced signal-to-noise performance. More details about the fabrication process may be found in Rando et al 2000.
The resolving power of the array as a function of wavelength represents a considerable improvement over S-Cam 1, although it is still some way below the intrinsic resolving power which would be expected in the absence of IR contamination or electrical noise. Contributions to the degradation in the resolving power in the actual array include spatial non-uniformities in the detector responsivity, noise in the Front-End electronics and by statistical fluctuations in the background radiation - essentially IR radiation originating from objects close to or in the field of view of the detector.