Previous: C. Coming Prepared with Good Coordinates.
When using the south guide probe at the 4-m, it is necessary to keep the probe further than plus or minus 70mm in x, and further than -50 in y. Otherwise the RC slit will be vignetting. When using slitlet masks, the probe must be kept further than 70mm in both x and y. Recall that when using the Risley prisms, the probe must be closer than 125mm or focusing the probe will be impossible.