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Flat fields can be obtained using either the twilight sky or the calibration system in the Cassegrain A&G.

Table 8 lists several areas of sky containing relatively few stars, well-suited for twilight sky flat-field observations. These are observed in exactly the same way as any other object.

For flatfield observations using the comparison lamps in the Cassegrain A&G, the first step is to put the acquisition and comparison mirror must be in, since the calibrating light is reflected from its bottom surface. Then switch the tungsten lamp on, select the required filter and take the exposure. The commands to do all this are as follows:

Exposure count-rates for flatfields using the tungsten lamp, the standard Harris broadband filters, and the f/4 camera are summarised in Table 9. This data can easily be scaled to observations with the f/2.1 camera, by multiplying the count rates by a factor of 4.

Thu Apr 7 00:29:52 BST 1994