  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.
