Telescope — A telescope is an instrument composed of one or more lenses or mirrors to gather and focus electromagnetic radiation.
Telescopes increase the observed angular size of objects, as well as their apparent brightness. The largest telescopes are used in astronomy. A telescope was first turned on the sky by Galileo Galilei, the Italian scientist. (Telescopes used for non-astronomical purposes may be called “transits,” “monoculars,” “binoculars,” “camera lenses,” or “spyglasses”.)
The word telescope alone usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation.
Radio telescopes are focused radio antennas, usually shaped like large dishes. The dish is sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Radio telescopes are often operated in pairs, or larger groups to synthesize large apertures. The current record is nearly the width of the Earth. Aperture synthesis is now being applied to optical telescopes.
X-ray and gamma-ray telescopes have a problem because the rays go through most metals and glasses. They use ring-shaped “glancing” mirrors made of heavy metals, that reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola.
The rest of this article considers optical telescopes.
Telescopes in which the primary light-gathering surface is a lens are called refractive telescopes, or “refractors,” and those in which it is a mirror are reflective telescopes, or “reflectors.” Refractors are similar in basic design and function to microscopes, and share a history with them.
The basic scheme is that the primary light-gathering element, called the “primary,” focuses light to a focal plane where it forms a bright virtual image. An “eyepiece” then magnifies the virtual image. Many types of telescopes fold the optical path with secondary or tertiary mirrors, usually to make the telescope more compact and reduce the width of its field of view.
The angular resolution of a telescope is determined primarily by the width of the objective, termed its “aperture.” Recently, it has become practical to perform aperture synthesis with optical telescopes. Increasingly, high-resolution optical telescopes are actually groups of widely-spaced smaller telescopes, linked together by carefully-controlled optical paths.
The sensitivity of a telescope is determined by both the area of its objective, and the sensitivity of the sensor. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.
Finally, the f-ratio of a telescope denotes how wide an angle the telescope can view. Low f-ratios indicate wide fields of view. Wide-field telescopes are used to track satellites and asteroids, for cosmic-ray research, and for surveys of the sky.
Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:
— 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 colors travels through a medium other than vacuum at different speeds. This causes chromatic aberration.
— There are technical difficulties involved in manufacturing and manipulating large-aperture lenses. One of them is that all real materials sag in gravity. A lens can only be held by its perimeter. A mirror, on the other hand, can be supported by the whole side opposite to its reflecting face.
Names of types:
— The simple refractor has a compound primary lens, and an eyepiece. The primary has two pieces of glass of different densities, “crown” and “flint” glass. Each side of each piece is ground and polished, and then the two pieces are glued together. The curvatures are designed to cancel chromatic aberration and spherical aberration.
— A monocular is a refractor, except that the optical path is folded with prisms to make a more compact telescope.
— Binoculars are just two monoculars mounted side-by side with adjustments to let both be used. A major practical advantage of these telescopes is not magnification, so much as a brighter field of view at dusk and dawn. Monoculars and binoculars with built-in compasses are used by army artillery units and ships to navigate by triangulating from topographic (shore) features. Hand-held telescopes are limited by hand-shaking to about 7 power. The brightest-field, best-magnifying practical monocular is about 7×50.
— 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-Chrtien 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-Chrtien design. It was invented by George Willis Ritchey and Henri Chrtien in the early 1910’s.
— 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-Chrtien telescopes for professional use are Schmidt cameras. These instruments have a very wide field a sharp focus, about 30 times greater than Ritchey-Chrtien, 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 abberations but avoid all the others. The spherical abberation 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.