Three mirror anastigmat survey telescope optimization

We investigate practical implementations of the Korsch(1) (1977) annular field three mirror anastigmat (AFTMA) telescope. This TMA offers a wide, diffraction-limited field of view (FOV) suitable for space-based survey missions. The advantages of this configuration include superior stray light baffling (distinct exit pupil) and the wide range of available focal lengths available from geometrically similar configurations. Key in the design of an AFFMA is the location of the Cassegrain focus (CF) and the exit pupil (EP). We investigate the effects of telescope geometry and optimization constraints on the location of the CF and EP, and suggest a family of geometries suitable for practical and vignetting-free AFTMA designs. The space of practical designs is generalized in terms of mirror diameter, telescope focal ratio and field of view for an annular, detector-tiled focal plane. Focal ratios between roughly f/10 and f/25 produce a geometric blur of less than 0.1 arcsec. For this range of f/numbers, the focal plane should be less than 1/2 the primary mirror (PM) diameter, and the cross-axis should be located roughly 0.6 PM diameters behind the PM vertex.