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A calibration system is provided consisting of an integrating sphere into which light is fed directly from two hollow cathode lamps ( normally Cu-Ar and Cu-Ne) and a Tungsten lamp for a red continuum source. Light from a further 6 lamps (Fe-Ar, Fe-Ne, Th-Ar, Al/Ca/Mg-Ne, Na/K-Ne, and Deuterium) is imaged via fused silica lenses onto 3 mm diameter fused silica light guides.
It should be noted that the transmission of the fused silica light guides is poor (a few per cent at best) and in general only the directly fed lamps are used. It is possible to exchange lamps, in order to feed light from any of the lamps listed above directly.
Any combination of lamps may be used simultaneously. The exit pupil of the integrating sphere is fitted with an obscuring disk to simulate the telescope entrance aperture obscuration, i.e. the secondary mirror structure. The reverse side of the acquisition mirror is used to feed the calibration light to the instrument. This enables (in principle !) simultaneous object acquisition and spectral calibration.
Two eight-position filter wheels are provided for the comparison system. The filters available are summarised in Table 2