New 4-mirror optical concept for very large telescopes with spherical primary and secondary mirrors giving excellent field and obstruction characteristics

摘要

The 10 m Keck project has demonstrated that very large segmented primaries can be actively controlled. But there are important technical and above all cost advantages accruing from a spherical primary. This presents, however, a serious problem for optical design solutions with adequate field performance, reasonable central obstruction and good baffling. Two new compact solutions are presented using 4 mirrors with optical power, the primary (f/1.5) and secondary both being spherical. Only the last mirror is strongly deformed (hyperbolic), the third being a weak hyperbola. Studies for the Gemini project and elsewhere have made it clear that technical solutions for improving the reflectivity of large optics will soon be available, making optical design solutions with 4 or more reflections acceptable. The f/no of the final image is about f/6 to f/7.3. The image is only slightly curved. The geometrical basis of the solutions is the use of a second optical axis at right angles, produced by a small folding flat.