UV/optical imaging spectroscopy with cryogenic detectors

Recently, the photon counting capabilities of cryogenic detectors have been extended into the UV/visible/NIR spectral region. Superconducting Tunnel Junctions (STJ) as well as Transition Edge Sensors (TES) have shown single-photon detection with moderate energy resolution well beyond λ=1 μm. They provide an excellent detection efficiency in the UV and visible (>50%) and high count rate capabilities. This allows for spectroscopic studies of weak sources or time-resolved studies of transient phenomena. These qualities make these detectors interesting alternatives to CCDs and micro-channel plates which are now the most commonly used detectors in UV/visible astronomy. While the present spectroscopic performance of single pixel STJs and microcalorimeters is already useful, it can be further enhanced by using lower Tc materials. The imaging properties, however, need to be much improved. Small prototype arrays of close-packed single STJs have been demonstrated, but large format arrays of individually connected STJs or microcalorimeters would be hampered by lack of cooling power, wire connection problems and massive electronics. Some solutions to this problem are discussed. An additional problem area is the detectorʹs sensitivity in the IR: a large load of thermal IR radiation (e.g. from warm optics) will seriously degrade the spectroscopic performance. Sophisticated filtering is required to prevent this without loss in sensitivity.