INFORMATION ABOUT TELESCOPES

OPTICAL TELESCOPES

Generally about optical telescopes

An optical telescope gathers and focuses light mainly from the visible part of the Electromagnetic spectrum (although some work in the infrared and ultraviolet). Thereby also pictures of the objects that the light originates from will be focused. Optical telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. Telescopes work by employing one or more curved optical elements - lenses or mirrors - to gather light or other electromagnetic radiation and bring that light or radiation to a focus where the image can be observed, photographed or studied.

Practical telescopes have at least two main lenses or lens collections. The objective resceives the light and focuses it. The oculular or eyepiece enlarges the image of the object further. The objective will make the image up- to down and right to left. To counteract this effect, many telescopes have a third element between to convert back.

The focusing can be done with convexe lenses, that is lenses that are thickest in the mid. They can also be done by concave mirrors, that is mirrors that bulge inwards at the side that collects the light. A combination of lenses and mirrors can also be used.

It is also possible to use mirrors to let the light go forth and back through the same tube. In this way the telescope can be made a lot shorter. There are three main types of telescopes.

The refracting telescope in which the light goes only through a collection of lenses to be focused after having passed.
- The reflecting telescope which uses only an arrangement of mirrors that eventually reflects the light onto a plane.
- The catadioptric telescope which uses a combination of mirrors and lenses.

Optical telescopes are used for astronomy and in many non-astronomical instruments including theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses, and spyglasses.

Refractor telescopes

A typical refractor has two basic elements, a convex objective lens and an eyepiece lens. The objective in a refracting telescope refracts or bends light at each end using lenses. This refraction causes parallel light rays to converge at a focal point; while those which were not parallel converge upon a focal plane. This can enable a user to view the image of a distant object as if it were brighter, clearer, and/or larger. Refracting telescopes can come in many different configurations to correct for image orientation and types of aberration.

Galilean telescope

The original design Galileo came up with is commonly called a Galilean telescope. It uses a convex objective lens and a concave eyepiece lens.

The first telescope used the same principles that all telescopes would rely upon. The combination of the two lenses gathered more light than the human eye could collect on its own, focused it, and formed an image. Because the image was formed by the bending of light, or refraction, these telescopes came to be known as refracting telescopes or, simply, refractors.

Galileo’s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens, the images were blurry and distorted. But it was good enough for Galileo to explore the sky.

Keplerian Telescope

The Keplerian Telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design. It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design but to overcome aberrations the simple objective lens needs to have a very high f-ratio (Johannes Hevelius built one with a 45 m (150 ft.) focal length). The design also allows for use of a micrometer at the focal plane (used to determining the angular size and/or distance between objects observed).

Achromatic telescope

The achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall although it was independently invented and patented by John Dollond. The design limits the effects of chromatic and spherical aberration by using an objective made of two pieces of glass (with different dispersion), "crown" and "flint glass". Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.

Apochromatic refractors Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (secondary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Such telescopes are sold in the high-end amateur telescope market. Apochromatic refractors are available with objectives of up to 553mm in diameter, but most are between 80 and 152mm.

Reflector telescopes

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.

Mirrors eliminate chromatic aberration but still produce other types of aberrations: In general, on axis they produce spherical aberration - the outer and inner zones of the telescope do not share a common focus. This was the construction flaw in the Hubble Space Telescope mirrors. Spherical aberration can be eliminated with aspheric (non-spherical) mirrors. Off axis, additional aberrations will become apparent:

Coma

a variation of telescope magnification with radial zone on the mirror typically appears as a radial smudging of the images which gets worse at the edges of the field. Spherical aberration and coma are eliminated in two mirror Ritchey Chretien designs. The best image plane is in general curved, which may not correspond to the detector's shape and leads to a focus error across the field.

Astigmatism is an azimuthal variation of focus around the aperture. Near the center of the field astigmatism is not usually a problem, but it gets rapidly worse once it becomes apparent - it varies quadratically with field angle.

Distortion over the field of view

face="Verdana">Distortion does not affect image quality (sharpness) but does affect object shapes. It can be corrected by image processing.

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.

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.

Catadiopteric telescopes

Catadioptric telescopes are designs that combine specifically shaped mirrors and lenses to allow very fast focal ratios (when used at the prime focus), while controlling coma and astigmatism.

Telescope makers also use catadioptric designs for any or all of the following reasons:

They employ spherical surfaces that are easier to manufacture.
When used in a cassegrain configuration it results in a long focal length instrument that is "folded" into a much smaller package.
Catadioptric designs are low maintenance and rugged since some or all of their elements are fixed in alignment (collimation).
Combining a moving primary mirror with a cassegrain configuration allow for large movements in the focal plane to accommodate cameras and CCDS.
The corrector plates seal the tube assembly from dust and dirt. They also block air currents from the interior of the tube, thereby increasing image stability.
A disadvantage to this design is that the secondary mirror blocks a portion of the light entering the tube.


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.

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 1960s. 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

Light path in a Maksutov-CassegrainMain article: Maksutov telescope
The Maksutov-Cassegrain is a variation of the Maksutov telescope, invented by Dmitri Maksutov. 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. The key difference from the similar Schmidt telescope design is the meniscus-shaped corrector plate, that has easy-to-make spherical surfaces, and not the complex aspherical form of the Schmidt design. Maksutovs tend to have a narrower field of view than Schmidt-Cassegrains due to their longer focal length and are generally heavier as well. However, their small secondary mirror gives them better resolution than a Schmidt-Cassegrain.

Picture of a Schmidt-Cassegrain telescope



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RADIO TELESCOPES

face="Verdana">Radio telescopes are directional radio antennae that often have a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Multi-element Radio telescopes are constructed from pairs or larger groups of these dishes to synthesize large "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. As of 2005, the current record array size is many times the width of the Earth, utilizing space-based Very Long Baseline Interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and Aperture Masking Interferometry at single reflecting telescopes.

X-ray and gamma-ray telescopes

X-ray and gamma-ray telescopes have a problem because these rays go through most metals and glasses. Some x-ray telescopes 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. Gamma-ray telescopes give up on focusing entirely, and use coded aperture masks; the pattern of shadows the mask creates can be reconstructed to form an image.

These types of telescopes are usually on Earth-orbiting satellites or high-flying balloons, since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum.

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