Reflecting telescope

A reflecting telescope (reflector) is an optical telescope which uses a combination of curved and plane (flat) mirrors to reflect light and form an image, rather than lenses to refract or bend light to form an image. The Italian monk Niccolo Zucchi is credited with making the first reflector in 1616, but his inability to shape the concave mirror accurately and the lack of means of viewing the image without blocking the mirror, caused Zucchi to give up on the idea. It was another 54 years before British scientist Sir Isaac Newton implemented 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 reflecting telescope is known as a Newtonian reflector.

While the Newtonian focus design is 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+.

Technical considerations

A curved 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 produce 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 in place 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

Newtonian focus

The Newtonian usually has a paraboloid primary mirror but for small apertures, say 12cm or less, if the focal ratio is 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 or field derotator is needed. On an equatorial mount, the telescope rotates around an axis parallel to the Earth's axis, allowing it to follow the apparent motion of stars. Field derotators can adjust the image of an altitude-azimuthally mounted telescope electronically, by linking to a guidance computer.

Cassegrain focus

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.

Vixen produce an 8 inch aperture modified Cassegrain design they refer to as a VISAC (Vixen Sixth-Order Aspheric Cassegrain). It has a fixed primary mirror with an open tube rather than a corrector plate and provides correction of aberrations via lenses in the draw tube of the focusser. The design has no coma and exceeds Ritchey-Chrétien performance by also addressing field curvature while being cheaper to produce. This particular design is also unusual in that it is a Cassegrain design but has a refractor style rack and pinion focuser.

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.

Thousands of amateur astronomers have purchased and used Schmidt-Cassegrain telescopes, with diameters from 20 cm (8 in.) to 48 cm (16 in.), since this type of telescope was introduced by Celestron in the 1970s. Now many companies mass-produce this type of telescope, at prices that make them quite affordable for many amateurs. One of the major advantages of the Schmidt-Cassegrain is that its folded light path makes the optical tube very short and squat, thus increasing its portability. It also has optics that are good for both planetary and deep sky observing.

Maksutov-Cassegrain

A well regarded luxury telescope using a Maksutov design is the Questar. It directs light from a "finder" scope and the main scope to the same eyepiece. It has a clear-aperture Maksutov reflector as the main telescope while the finder is a 1 inch refractor. The focal plane of the reflector and refractor are the same (the refractor probably has a factory adjustment). A flat-mirror near the bottom reflects light to the finder's primary, and a movable mirror at the back of the larger cassegrain's hole switchs the optical path of the large telescope between the eyepiece and the camera attachment on the back. When the camera is engaged, the finder-scope is operational.

Vixen produce 8, 10.25 and 13 inch aperture modified Maksutov-Cassegrain design. It has an open tube rather than a corrector plate and provides correction of aberrations via a two element miniscus corrector lens in front of the secondary. This design was originally envisaged by G. I. Popov with a practical implementation by Yu. A. Klevtsov. The 8 inch employs a refractor style rack and pinion focuser while the larger apertures move the primary mirror as in most other Cassegrain designs.

Gregorian

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

Dall-Kirkham

The Dall-Kirkham telescope design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a Cassegrain or Ritchey-Chretien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15.

Celestron produce a fast f/6.8 astrograph based on a modified Dall-Kirkham design which is said to address the off-axis coma problems of this design. Takahashi produce a folded Dall-Kirkham design called a Mewlon with apertures of 7" to 12" and focal ratios around f/12. Through the use of a field flattener they have achieved focal ratios as low as f/9.

Schmidt camera

The Schmidt camera, invented by Bernhard Schmidt, is not technically a telescope since the light path does not exit to an eyepiece. This is because the image formed by the lens and mirror lies along a curved rather than flat focal plane, so it cannot be viewed with 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.

Focal planes

Prime focus

In a prime focus design in large observatory telescopes, 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.

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