A comparison of designs of off-axis Gregorian telescopes for mm-wave   large focal plane arrays
  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. 
Shaul Hanany, Daniel P. Marrone (School of Physics and Astronomy,   University of Minnesota/Twin Cities)