Analysis of astronomical data from optical superconducting tunnel junctions

Currently operating optical superconducting tunnel junction (STJ) detectors, developed in ESA, can simultaneously measure the wavelength (delta lambda = 50 nm at 500 nm) and arrival time (to within ~5 micros) of individual photons in the range 310-720 nm with an efficiency of ~70%, and with count rates of order 5,000 photons per second per junction. A number of STJ junctions placed in an array format generates four-dimensional data: photon arrival time, energy, and array element (X,Y). Such STJ cameras are ideally suited for, e.g., high time- resolution spectrally-resolved monitoring of variable sources or low-resolution spectroscopy of faint extragalactic objects. The reduction of STJ data involves detector efficiency correction, atmo- spheric extinction correction, sky background subtraction, and, unlike that of data from CCD-based systems, a more complex energy calibration, barycentric arrival time correction, energy range selection, and time binning; these steps are, in many respects, analogous to procedures followed in high-energy astro- physics. This paper discusses these calibration steps in detail using a repre- sentative observation of the cataclysmic variable UZ Fornacis; these data were obtained with ESA's S-Cam2 6x6-pixel device. We furthermore discuss issues re- lated to telescope pointing and guiding, differential atmospheric refraction, and atmosphere-induced image motion and image smearing (`seeing') in the focal plane. We also present a simple and effective recipe for extracting the evolu- tion of atmospheric seeing with time from any science exposure, and discuss a number of caveats in the interpretation of STJ-based time-binned data, such as light curves and hardness ratio plots.