Galileo revolutionized astronomy when he applied the telescope to the study of extra-terrestrial bodies in the early 17th century. Until then, magnification instruments had never been used for this purpose. Since Galileo’s pioneering work, increasingly more powerful optical telescopes have been developed, as has a wide array of instruments capable of detecting and measuring radiation in every region of the electromagnetic spectrum. The observational capability has been further enhanced by the invention of various kinds of auxiliary instruments (e.g., the camera, spectrograph, and charge-coupled device) and by the use of electronic computers, rockets, and spacecraft in conjunction with telescope systems.

Pritish Kumar gives an illustration of the Telescope, a device used to form magnified images of distant objects.

Refracting telescopes

Commonly known as refractors, telescopes of this kind are typically used to examine the Moon, other objects of the solar system such as Jupiter and Mars, and binary stars. The name refractor is derived from the term refraction, which is the bending of light when it passes from one medium to another of different density—e.g., from air to glass. The glass is referred to as a lens and may have one or more components. The physical shape of the components may be convex, concave, or plane-parallel. This diagram illustrates the principle of refraction and the term focal length.

The focus is the point, or plane, at which light rays from infinity converge after passing through a lens and traveling a distance of one focal length. In a refractor the first lens through which light from a celestial object pass is called the objective lens. It should be noted that the light will be inverted at the focal plane. A second lens, referred to as the eyepiece lens, is placed behind the focal plane and enables the observer to view the enlarged, or magnified, image. Thus, the simplest form of refractor consists of an objective and an eyepiece, as illustrated in the diagram.

Light gathering and resolution

The most important of all the powers of an optical telescope is its light-gathering power. This capacity is strictly a function of the diameter of the clear objective—that is, the aperture—of the telescope. Comparisons of different-sized apertures for their light-gathering power are calculated by the ratio of their diameters squared; for example, a 25-cm (10-inch) objective will collect four times the light of a 12.5-cm (5-inch) objective ([25 × 25] ÷ [12.5 × 12.5] = 4). The advantage of collecting more light with a larger-aperture telescope is that one can observe fainter stars, nebulae, and very distant galaxies.

Resolving power is another important feature of a telescope. This is the ability of the instrument to distinguish clearly between two points whose angular separation is less than the smallest angle that the observer’s eye can resolve. The resolving power of a telescope can be calculated by the following formula: resolving power = 11.25 seconds of arc/d, where d is the diameter of the objective expressed in centimetres. Thus, a 25-cm-diameter objective has a theoretical resolution of 0.45 second of arc and a 250-cm (100-inch) telescope has one of 0.045 second of arc.

An important application of resolving power is in the observation of visual binary stars. There, one star is routinely observed as it revolves around a second star. Many observatories conduct extensive visual binary observing programs and publish catalogs of their observational results. One of the major contributors in this field is the United States Naval Observatory in Washington, D.C.

The Schmidt telescopes

The Ritchey-Chrétien design has a good field of view of about 1°. For some astronomical applications, however, photographing larger areas of the sky is mandatory. In 1930 Bernhard Schmidt, an optician at the Hamburg Observatory in Bergedorf, Germany, designed a catadioptric telescope that satisfied the requirement of photographing larger celestial areas. A catadioptric telescope design incorporates the best features of both the refractor and the reflector—i.e., it has both reflective and refractive optics.

The Schmidt telescope has a spherically shaped primary mirror. Since parallel light rays that are reflected by the center of a spherical mirror are focused farther away than those reflected from the outer regions, Schmidt introduced a thin lens (called the correcting plate) at the radius of curvature of the primary mirror. Since this correcting plate is very thin, it introduces little chromatic aberration. The resulting focal plane has a field of view several degrees in diameter. The diagram illustrates a typical Schmidt design.

Multimirror telescopes

The main reason astronomers build larger telescopes is to increase light-gathering power so that they can see deeper into the universe. Unfortunately, the cost of constructing larger single-mirror telescopes increases rapidly—approximately with the cube of the diameter of the aperture. Thus, in order to achieve the goal of increasing light-gathering power while keeping costs down, it has become necessary to explore new, more economical and non-traditional telescope designs.

The two 10-metre (33-foot) Keck Observatory multimirror telescopes represent such an effort. The first was installed on Mauna Kea on the island of Hawaii in 1992, and a second telescope was completed in 1996. Each of the Keck telescopes comprises 36 contiguous adjustable mirror segments, all under computer control. Even-larger multimirror instruments are currently being planned by American and European astronomers.

Solar telescopes

Either a refractor or a reflector may be used for visual observations of solar features, such as sunspots or solar prominences. Special solar telescopes have been constructed, however, for investigations of the Sun that require the use of such ancillary instruments as spectroheliographs and coronagraphs.

These telescopes are mounted in towers and have very long focus objectives. Typical examples of tower solar telescopes are found at the Mount Wilson Observatory in California and the McMath-Hulbert Observatory in Michigan. The long focus objective produces a very good scale factor, which in turn makes it possible to look at individual wavelengths of the solar electromagnetic spectrum in great detail.

A tower telescope has an equatorially mounted plane mirror at its summit to direct the sunlight into the telescope objective. This plane mirror is called a coelostat. Bernard Lyot constructed another type of solar telescope in 1930 at Pic du Midi Observatory in France. This instrument was specifically designed for photographing the Sun’s corona (the outer layer), which up to that time had been successfully photographed only during solar eclipses.

The coronagraph, as this special telescope is called, must be located at a high altitude to be effective. The high altitude is required to reduce the scattered sunlight, which would reduce the quality of the photograph. The High-Altitude Observatory in Colorado has such a coronagraph. The principle has been extended to build instruments that can search for extrasolar planets by blocking out the light of their parent stars. Coronagraphs are also used on board satellites, such as the Solar and Heliosphere Observatory, that study the Sun.

Reference

https://www.britannica.com/science/optical-telescope/Reflecting-telescopes