INT Cassegrain (Figure 4)

The outer boundary of the guide probe field is set by the onset of vignetting (no vignetting at 10 arcmin radius; 50% at 11 arcmin, so rad) for the latter figure and the xy limits. The inner boundary of the field is set by the need to avoid obstructing the spectrograph slit with the guide probe, giving a rectangular excluded area. The full 4 arcmin length of the slit has been used in calculating the size of the inner bboundary.