Analysis of astronomical data from optical superconducting tunnel junctions

Abstract

Currently operating optical superconducting tunnel junction (STJ) detectors, developed at the European Space Agency (ESA), can simultaneously measure the wavelength (Dl550 nm at 500 nm) and arrival time (to within ;5 ms) of individual photons in the range 310 to 720 nm with an efficiency of;70%, and with count rates of the order of 5000 photons s 21 per junction. A number of STJs placed in an array format generates 4-D data: photon arrival time, energy, and array ele- ment (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, atmospheric extinction correction, sky background subtraction, and, unlike that of data from CCD-based systems, a more complex energy calibration, barycentric ar- rival time correction, energy range selection, and time binning; these steps are, in many respects, analogous to procedures followed in high- energy astrophysics. We discuss these calibration steps in detail using a representative observation of the cataclysmic variable UZ Fornacis; these data were obtained with ESA's S-Cam2 636-pixel device. We furthermore discuss issues related to telescope pointing and guiding, differential atmospheric refraction, and atmosphere-induced image mo- tion and image smearing (''seeing'') in the focal plane. We also present a simple and effective recipe for extracting the evolution 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. © 2002 Society of Photo-Optical Instrumen-