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The relative geometry of the slit and the autoguider field is illustrated in figure 2. The autoguider has an azimuthal travel of 180 degrees, but use of the first 35 degrees of this will result in the autoguider probe vignetting the slit viewing optics. The azimuthal co-ordinate of the autoguider is specified in millidegrees, and the radial co-ordinate in microns, and they are related to the distance from the guide star from the slit centre, and the position angle of the guide star relative to the slit centre, by:--
Where and are the co-ordinates of the autoguider probe in the frame of the A&G box; and are the distance and sky position angle of the guide star relative to the slit; and is the sky position angle of the slit.
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