  We compare the diffraction-limited field of view (FOV) provided by four types
of off-axis Gregorian telescopes: the classical Gregorian, the aplanatic
Gregorian, and designs that cancel astigmatism and both astigmatism and coma.
The analysis is carried out using telescope parameters that are appropriate for
satellite and balloon-borne millimeter and sub-millimeter wave astrophysics. We
find that the design that cancels both coma and astigmatism provides the
largest flat FOV, about 21 square degrees. We also find that the FOV can be
increased by about 15% by optimizing the shape and location of the focal
surface.
