Reflecting telescope: Difference between revisions

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 chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses. The traditional two-mirrored facedtelescope 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 2 meters or more.

A curved primary mirror is the reflector telescope's basic optical element and creates an image

Reflector mirrors eliminate chromatic aberration but still produce other types of aberrations:

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

There are reflector designs and modifications such as catadioptrics that correct some of these aberrations.

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 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.

{{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.

• Schmidt-Newton telescope

The Cassegrain (sometimes called the "Classic 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 the cost of some loss of light-gathering power.

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 Chrétienan 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 classic 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.

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.

• Schmidt-Cassegrain telescope
• Schmidt camera
• Maksutov telescope (Cassegrains)

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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.

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.

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. This is the design used in a Nasmyth telescope.

• List of largest optical reflecting telescopes
• Large liquid mirror telescope
• Catadioptric
• [[