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Slit unit and dekker slide

The UES slit has a maximum length of 67.3 mm (5 arcmin), corresponding to the full field transmitted by the Nasymth image derotation optics. The slitlength actually used is defined by the dekker slide or by the autoguider mirror (see section 16.2).

The slit width can be continuously varied between .02 and 20 mm (0.09 to 90 arcsec).

The entire slit assembly can be rotated by 12 degrees. Note that the reason for rotating the slit assembly is not to change the PA of the slit on the sky; this is the function of the image derotation optics (see Part iv). The reason for rotating the slit assembly is that for a fixed slitangle, the slit as projected on the detector will appear to rotate as a function of observing wavelength. It is therefore necessary physically to rotate the slit assembly in order to keep the projected slit aligned with the rows/columns of the detector.

A shutter is mounted at the rear of the slit. This is a fast shutter, used for timing CCD exposures. The shutter used is a Prontor magnetic shutter E/100, with a specified maximum opening/closing time of 34 msecs.

At the front of the slit assembly is a dekker slide, which contains a mask used to define the slit length. The dekker mask, usually referred to as the 'Concorde dekker' is illustrated in Fig. 2. The shape of this mask allows the slitlength to be set to a high degree of accuracy over the range from 0.33 mm (1.5 arcsec) to 4.62 mm (20 arcsec), and a rather coarser setting of slitlength over the range from 4.62 mm (20 arcsec) to 67 mm (5 arcmin).

Mon Mar 14 16:50:31 GMT 1994