New optical solutions for very large telescopes using a spherical primary.

Details New optical solutions for very large telescopes using a spherical primary. New optical solutions for very large telescopes using a spherical primary.

Feb 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...294..322w&link_type=abstract

Astronomy and Astrophysics 294, 322-338 (1995)

Astronomy and Astrophysics

Astrophysics

3

Telescopes

Scientific paper

The Keck 10m telescope has shown that segmented primaries can be successfully controlled actively in very large sizes. But there are important technical and above all cost advantages accruing from a spherical primary, particularly if it is made very fast to give compact solutions. From Schwarzschild theory, at most three further aspheric mirrors will be sufficient to achieve field correction with a spherical primary. In single-axis solutions, there are acute problems of obstruction for useful fields unless at least one of the supplementary mirrors is made too large to make the solution attractive for very large telescopes. The obstruction problem can be completely solved by a 2-axis solution, using a small supplementary plane mirror. We believe that modern high-efficiency reflecting coats for large optics will make it perfectly acceptable to have 4 mirrors with optical power and a small flat. Two basic solutions are possible using one or two powered mirrors disposed on the altitude axis of an alt-az telescope mount. This gives excellent field correction over +/-18arcmin with a conveniently placed and well-baffled final image at about f/7, effectively at a "quasi-Nasmyth" focus. Furthermore, the optical geometry of these solutions is so relaxed that the convex secondary can also be made spherical: two aspheric mirrors are sufficient. In both cases, the residual field curvature is small. A modified form with a further aspheric mirror gives a fast (f/2.8) version with a field extended to about +/-30arcmin. Fast, single-axis solutions are also given, but these are limited by the size of the supplementary mirrors.

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