In an alternate scheme, called a modified Cassegrain, a small optical flat placed immediately in front of the primary brings the light out to the side of the telescope tube and eliminates the need for a perforated primary. Like the Gregorian and Newtonian telescopes, the paraboloid-hyperboloid combination of the Cassegrain is free from spherical aberration.
The Dall-Kirkham telescope is a variant of the Cassegrain that uses a concave ellipsoidal primary mirror and a convex spherical secondary. Invented independently by the English amateur telescope-maker Horace Dall (1901–1986) and the American Alan Kirkham, it is particularly suited to planetary observation where good resolution is more important than wide field of view.
Related entry Schmidt-Cassegrain telescope
Related category TELESCOPE EQUIPMENT AND TECHNIQUES
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