A Quick Read
Two Mirror Reflector and Catadioptric Types
Catadioptric: A system using both lenses and mirrors
Full-Aperture Corrector: A Schmidt or Maksutov or Houghton type refractive corrector lens or lenses that sit at the front of the telescope and is the first element to receive light.
Near Focal Corrector: A refractive smaller element or elements located near the termination of focus. Sometimes referred to as a downstream corrector.
Non-active Optic: A flat mirror or plane parallel window.
Reflecting Telescope Optical System Types
Basic Pure Type: Cassegrain - Two mirrors only. Concave paraboloidal primary and convex hyperboloidal secondary. Intrinsically or naturally a slow system. Mostly for high resolution use.
Primary Variations: All of the following are generally considered as Cassegrain types. 1. Dall-Kirkham - Two mirrors only. Must have convex spherical secondary and accompanying ellipsoidal primary. Specifically a slow, high resolution system. 2. Ritchey-Chretien - Two mirrors only. Concave hyperboloidal primary and convex hyperboloidal secondary. Extreme curves and aspherization turn this into a moderately wide field system corrected for coma. Originally developed for large telescopes. 3. Pressman-Camechiel - Two mirrors only. Concave spherical primary and convex oblate spherical secondary. Hideous off-axis aberrations. Very narrow usable field. Not much used.
Secondary Variations: 1. Corrected versions of all three above types, as in Corrected Dall-Kirkham. Has near focal corrector. Does not have a full-aperture corrector. Corrector typically fixes obtrusive coma in expanded wider field applications.
Basic Pure Type: Gregorian - Two mirrors only. Concave paraboloidal primary and concave ellipsoidal secondary. Intrinsically or naturally a very slow system due to extrafocal positioning of secondary. High resolution use.
Secondary Variations: 1. Corrected versions exist similar to Cassegrain. Basic design has variations similar to all three Cassegrain types. Does not have a full-aperture corrector.
Basic Pure Type: Full-aperture Corrector or Schmidt type. A catadioptric system. One primary mirror only with spherical or varying aspheric surface. Prime focus or Newtonian configuration. Developed and used for wide-field and extreme wide-field photography.
Primary Variations: 1. Schmidt - Aspheric single or double element, thin full-aperture corrector in combination with a spherical mirror. Corrector is at or near the radius of curvature. 2. Maksutov - Single element spherical, thick full-aperture corrector in combination with a spherical or slightly aspherized mirror. Corrector is at or near the focus. 3. Houghton - Two or three element spherical, thin full-aperture corrector in combination with a spherical or slightly aspherized mirror. Corrector is at or near the focus.
Secondary Variations: 1. Wright - A Schmidt system with the corrector at or near the focus. Mirror is an oblate sphere. Not as well corrected as the Schmidt version but has a shorter tube length. 2. Houghton systems can have corrector near the radius of curvature. Baker-Schmidt - Intermediary correctors to correct for ultrawide-field use.
Basic Pure Type: Near-focal Corrector type. A catadioptric system - One primary mirror only with spherical or varying aspheric surface with one or more near-focal corrector refractive elements. Prime focus or Newtonian configuration. Developed and used for wide-field and moderately wide-field photography.
Primary Variations: Corrected Basic Cassegrain and Schmidt Cassegrain types
Hybrid Systems: 1. Schmidt-Cassegrain - A two mirror system having mirrors of any aspheric configuration that uses a full-aperture corrector. Such a system can never be called a Ritchey-Chretien, it is rather a modified Schmidt-Cass where the primary and/or secondary mirrors are aspherized. A Schmidt-Gregorian is possible. Can be made faster than a simple two mirror system because spherical correction is accomplished by means of the corrector and primary and secondary mirrors need not be aspherized, though aspherizing the mirrors improve performance for off-axis images in very fast, wide-field systems.