Reflecting telescope
A reflecting telescope is an
optical telescope which uses a combination of curved and plane
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.
Encyclopedia
A
reflecting telescope is an
optical telescope which uses a combination of curved and plane
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 2 meters or more.
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:
- spherical aberration when a non-parabolic mirror is used
- 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 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.
- Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.
- 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
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
amateur telescope makers as a home-build project.
See also
Cassegrain
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 the cost of some loss of light-gathering power.
Ritchey-Chrétien
The
Ritchey-Chrétien is a specialized Cassegrain reflector which has two hyperbolic mirrors . 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.
Dall-Kirkham
The
Dall-Kirkham cassegrain telescope's 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 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.
Schiefspiegler
An unusual variant of the Cassegrain is the
Schiefspiegler telescope , 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.
See also:
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.
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.
Nasmyth and Coudé focus
The
Nasmyth 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. Adding further optics that deliver the light to a fixed focus point that does not move as the telescope is reoriented gives you a
Coudé focus. This design is often used on large observatory telescopes, as it allows heavy observation equipment, such as
spectrographs, to be more easily used.
See also
