Two aspheric reflecting elements can correct exactly the spherical aberration and coma of a spherical primary mirror. The remaining off-axis aberrations depend mainly on the sizes of the secondary and tertiary mirrors. Two solutions are presented in which the secondary mirror is 1 meter in diameter. The first is for a 5-meter f/0.8 primary suitable for use as an optical Arecibo (fixed primary) telescope. The second is for a fully steerable telescope with a 12-meter f/1.33 primary. In both cases, 1 arcsecond images are formed at the edge of a field 4 arcminutes in diameter. The collimation tolerances for the corrector assemblies are stringent in comparison to most conventional telescopes, but modest in comparison to the precision required to maintain a segmented primary mirror.
Stephen A. Shectman,
"Reflecting Correctors For Large Spherical Primaries", Proc. SPIE 0444, Advanced Technology Optical Telescopes II, (3 November 1983); doi: 10.1117/12.937964; https://doi.org/10.1117/12.937964