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Moving along the main beam from the telescope to spectrograph slit, the first component encountered in the A&G unit is the autoguider mirror. This has a pivotal role in the operation of the A&G unit, since which of the possible lightpaths is operational is determined primarily by moving the mirror.
The mirror is inclined at an angle of 45 degrees to the lighpath, so that light coming from the telescope and intercepted by the mirror is directed downwards. This light comes to a focus below the mirror, in what is normally referred to as the 'reflected focal plane'; this reflected focal plane is used for acquisition and guiding, as described in section 16.4. Holes are cut in the mirror, in order to allow the central part of the beam to pass through the mirror and reach the spectrograph, as described in section 16.3; in normal operation the inner part of the field, which contains the object being observed, is therefore visible at the slit, whilst light from the outer part of the field is directed to the reflected focal plane for offset autoguiding.
The size of the hole in the autoguider mirror determines the field of view visible at the slit. There are three holes with different sizes, giving different tradeoffs between field of view at the slit and the amount of field available in the reflected focal plane for autoguiding. The required hole is selected by moving the mirror. It is also possible to move the mirror to a position with no hole, where all the light is directed to the reflected focal plane, and to a clear position where all the light reaches the spectrograph slit.
These options are summarised in Tab. 20. This gives the unvignetted and partially vignetted field of view at available at the slit for each of the options. Figs. 9i and 9iishow plots of the vignetting along the slit produced by the autoguider mirror in the different positions.
autoguider mirrors is a mirror which, if selected (see ICL command UMIRROR ULAMPS), directs light from the calibration unit into the spectrograph (see section 16.7). When the autoguider is moved to its other extreme position, the return light path to the acquisition TV is clear (see sec. 16.6 and also ICL command UTVVIEW).