Reflecting telescope

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Ritchey 24" reflecting telescope

A reflecting telescope (reflector) is an optical telescope which uses mirrors, rather than lenses, to reflect light. The British scientist Sir Isaac Newton designed the first reflector circa 1670. He designed the reflector in order to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses. The traditional two-mirrored reflector is known as a Newtonian reflector.

While the Newtonian focus design still used in amateur astronomy, professionals now tend to use prime focus, Cassegrain focus, and coudé focus designs. By 2001, there were at least 49 reflectors with primary mirrors having diameters of 2m+.

Contents

1 Technical considerations

2 Reflecting telescope designs

2.1 Schmidt camera
2.2 Newtonian focus
2.3 Cassegrain focus

2.3.1 Ritchey-Chrétien
2.3.2 Schmidt-Cassegrain
2.3.3 Maksutov
2.3.4 Gregorian

3 Focal planes

3.1 Prime focus
3.2 Coudé focus

4 See also

Technical considerations

The primary mirror is the reflector telescope's basic optical element and creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation.

Reflector mirrors eliminate chromatic aberration but still contain other types of aberrations.Expensive telescopes will have additional optical elements to correct some of these aberrations;

spherical aberration when a non-parabolic mirror is used (the image plane is not flat)
coma
distortion over the field of view

Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:

In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration in uncorrected lenses and creating an aberration-free large lens is a costly process. A mirror can eliminate this problem entirely.
There are structural problems involved in manufacturing and manipulating large-aperture lenses. A lens can only be held by its edge, which means that the sag due to gravity can be sufficient to distort the image. In contrast, a mirror can be supported by the whole side opposite its reflecting face.

Reflecting telescope designs

Schmidt camera

The Schmidt camera, invented by Bernhard Schmidt, is not technically a telescope since the light path does not exit to an eyepiece. Therefore it is strictly a camera, with a photographic plate, film or a CCD placed at the prime focus. The Schmidt camera corrects for spherical aberration by placing a correcting lens at the center of curvature of the mirror. The corrector, which is thicker in the middle and the edges, corrects the light paths so that the outer and inner parts of the mirror focus at the same distance. A simpler lensless Schmidt can be made by placing an aperture stop at the center of curvature, stopping the aperture to f/8 or longer. This degrades the light gathering ability of the telescope but produces a sharp image while preserving the wide field of the shorter focal length mirror. The 48" telescope at Palomar Observatory is actually a Schmidt camera.

Newtonian focus

The Newtonian usually has a paraboloid primary mirror but for small apertures, say 12cm or less, if the focal ratio if f/8 or longer a spherical primary mirror is sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at 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 as a home-build project. Since the light path is unfolded, the tube is often quite long and heavy. The difficulty in making the paraboloid mirror with accuracy is proportional to its diameter. Amateurs often begin by producing a mirror of modest size (up to six inches/15cm diameter) and progressing to larger sizes once they have some experience. Some amateurs produce a spherical mirror, and live with the spherical aberration, which is acceptable in longer focal length mirrors, where the difference between a spheroid and a paraboloid is very small. However, amateurs can grind and polish diffraction-limited paraboloid mirrors of substantial size (over 12 inches/300mm diameter). If straight spider vanes support the secondary mirror they cause diffractive effects making stars appear to "flare" in four or six directions--curved spiders can markedly reduce flares.

A Newtonian telescope placed on a simple altazimuth mounting is known as a Dobsonian. This variant (popularized by John Dobson in the 1970s) allows for a large primary mirror in a relatively cheap and lightweight telescope which is simple to build and use. For photographic use, an equatorial mount is needed, in which the telescope rotates around an axis parallel to the Earth's axis, allowing it to follow the apparent motion of stars.

Cassegrain focus

The Cassegrain has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design. 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 straight-vaned support spider. The closed tube stays clean, and the primary is protected, at some loss of light-gathering power.

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Light path in a Cassegrain

Ritchey-Chrétien

The Ritchey-Chrétien is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a flat focal plane, making it well suited for wide field and photographic observations. 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.

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

Schmidt-Cassegrain

The Schmidt-Cassegrain is a classic wide-field telescope. 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 correct the spherical aberration caused by the primary mirror. Thirty inch Schmidt-Cassegrains are used for sky surveys at astronomical observatories and satellite tracking stations.

Maksutov

The Maksutov, invented by Dmitri 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. Maksutovs have a narrower field of view than Schmidt-Cassegrains and are generally heavier as well. However, their small secondary mirror gives them better resolution than a Schmidt-Cassegrain.

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Light path in a Maksutov

One very popular luxury telescope design was the Questar. It ran a "finder" scope and the main scope to the same eyepiece. It had a 9cm clear-aperture 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 larger 10-cm cassegrain's 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.

An unusual variant of the Cassegrain is the Schiefspiegler telescope ("skewed" or "oblique reflector"), which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to several other aberrations that must be corrected.

Gregorian

The Gregorian telescope, invented by James Gregory, employs a concave, not convex, secondary mirror and in this way achieves a upright image, useful for terrestial observations. Whereas the design has largely fallen in disfavour, some small spotting scopes are still built this way.

Focal planes

Prime focus

In a prime focus design, the observer sits inside the telescope, at the focal point of the reflected light. In the past this would be the astronomer himself, but nowadays CCD cameras are used.

Radio telescopes often have a prime focus design. The mirror is replaced by a metal surface for reflecting radio waves, and the observer is an antenna.

Coudé focus

The Coudé design is similar to the Cassegrain except no hole is drilled in the primary mirror; instead, a third mirror reflects the light to the side, and further optics deliver the light to a fixed focus point that does not move as the telescope is reoriented. This design is often used on large observatory telescopes, as it allows heavy observation equipment, such as spectrographs, to be more easily used.