Reflecting Telescope

• W.M. Keck telescopes (Hawaii)

The Prime Focus

The Cassegrain Focus

The Coudé Focus

• The Newtonian has a parabolic primary mirror, and a flat secondary that reflects the focal plane to the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateurs. Since the light path is unfolded, the tube is quite long and heavy. The parabolic mirror is difficult to produce with accuracy. Some amateurs produce a spherical mirror, and live with the spherical aberration. The spider supporting the secondary mirror often introduce diffractive effects that cause stars to appear to "flare" in four or six directions.

• The Cassegrain has a spherical primary mirror, and a spherical secondary mirror that reflects the light back down through a hole in the primary. This is one of the most attractive designs. The folded optics make the telescope compact. The secondary corrects spherical aberration intruduced by the spherical primary. The secondary mirror introduces a diffraction pattern that seems to create a ring around stars. The spherical mirrors are easy to produce with automatic equipment. On smaller telescopes, and camera lenses, the secondary is often mounted on an optically-flat, optically-clear glass plate that closes the telescope tube. This support eliminates the "star-shaped" diffraction effects caused by a support spider. The closed tube stays clean, and the primary is protected, at some loss of light-gathering power.

• The Maksutov is similar to the Cassegrain. It starts with an optically transparent corrector lens that is a section of a hollow sphere. It has a spherical primary mirror, and a spherical secondary that is often just a mirrored section of the corrector lens. Maksutovs are mechanically simpler than small Cassegrains, have a closed tube and all-spherical optics.
  • One very popular luxury telescope design was the Celestron. It ran a "finder" scope and the main scope to the same eyepiece. It had a 10cm Maksutov reflector as the main telescope. The finder was a 2.5 cm refractor. The focal plane of the reflector and refractor were the same (probably the refractor had a factory adjustment). A flat-mirror near the bottom reflected light to the finder's primary, and a movable mirror at the back of the 10-cm cassegrain hole switched the optical path of the large telescope between the eyepiece and the camera attachment on the back. When the camera was engaged, the finder-scope was operational.

• The Ritchey-Chrétien is a specialized Cassegrain reflector with the advantage that it is coma free. This is why almost every professional reflector telescope in the world is of the Ritchey-Chrétien design. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910s.

• The Schmidt-Cassegrain is a classic wide-field telescope. 30 inch Schmidt-cassegrains are used for sky surveys at astronomical observatories and satellite tracking stations. The first optical element is a "schmidt corrector plate." The plate is figured by placing a vacuum on one side, and grinding the exact correction required to fix the spherical correction. The primary mirror is spherical.

• One exception to the supremacy of Ritchey-Chrétien telescopes for professional use are Schmidt cameras. These instruments have a very wide field a sharp focus, about 30 times greater than Ritchey-Chrétien, with the drawbacks that the focus is inaccessible, making them usable only as cameras, and contrary to Cassegrain, they have their physical length is at least twice they focal length. Their optical performance come from the use of a sperical mirror which reintroduce the spherical and field curvature aberrations but avoid all the others. The spherical aberration is overcome by using a corrector lens in front of the telescope at the radius of curvature of the mirror and field curvature are compensated with a film-holder that stretches the film into a mild spherical shape.