The guide field is a rectangle defined by the range of travel of the guide probe, with a smaller rectangle chopped out of one corner so as to avoid vignetting of the central region of the field. The additional condition excludes the region with: and , with x = 156.9 mm and y = 111.4 mm. Note that the ``encoder scale'' quoted for the JKT f/8 (WFC) configurations is not a bit size. The WFC guide probe is positioned manually and its location is read in mm.