SuperCOSMOS is housed in a special environment, for two reasons. First, the machine room and plate preparation room are a clean room environment in order to reduce the amount of dust adhering to the emulsion and optical surfaces. This is particularly important in the measurement of films, when electrostatic attraction of dust is a major problem.
Second, the machine room is temperature controlled in order to maintain the
mechanical stability of the machine, and hence the astrometric accuracy. The
high degree of repeatability required means that any physical expansion of
mechanical subsystems due to temperature fluctuations must be limited to
m. For glass components, such as the Moire fringe gratings, this
requires a temperature stability of 0.1K. As the absolute inaccuracies are
larger than this, then in order for software to be able to remove systematic
errors, that temperature stability must be maintained over long
timescales as well as during the scanning of an individual plate. Temperature
stability of the photographic plate is also desirable, as expansion or
contraction of the plate during the two hours needed per scan would cause
differential shearing of the apparent positions of stars which may be
difficult to remove from the astrometric solution.