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The UES must be maintained at a stable temperature to avoid degrading the instrumental resolution. In particular, temperature differences of a few tenths of a degree within the prisms will cause an unaccaptable variation in the spectral index. The prisms have a long thermal time constant, of order 24 hours, and so cannot be subjected to sudden changes in temperature.
The main body of the spectrograph (but not the spectrograph camera nor the A&G unit) is therefore contained within a thermal enclosure, consisting of a number of aluminium panels mounted on a metal frame. Each panel is maintained at a constant temperature by circulating a temperature-controlled fluid (a water--glycol mix) through pipes attached to the inside of the panel. The temperature of the fluid is stabilised by the Tamson unit, which first cools the fluid to below the desired temperature and then heats it by a precisely controlled amount; the reason for this approach is that it is much easier to heat a fluid by a precise amount than to cool it by a precise amount. The rate of flow within each panel can be independently adjusted by means of flowmeters, in order to equalise the flow through different panels.
In order to minimise the amount of heat dumped by the Tamson unit into the dome, the UES should not be kept at the same temperature throughout the year. Instead, the temperature should be changed slowly (i.e. on timescales much longer than the thermal time constant of the prisms), to follow seasonal variations in the dome temperature. Typically the enclosure temperature is adjusted to approximately equal the dome temperature averaged over a 24 hour period, hence 10C in the winter months, rising to 15C during the summer.
In order to keep the spectrograph optics dry and dust--free, the inside of the thermal enclosure and the spectrograph camera are both flushed with dry air. The temperature of the dry air is controlled by means of a heat exchanger to be equal to the temperature of the fluid circulating in the enclosure panels.