Telescopes.

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1 Telescopes A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an instrument designed for the observation of remote objects. The term usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation and for other signal types. Types of telescopes (1) 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). 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. Optical telescopes are used for astronomy and in many nonastronomical instruments including theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses and spyglasses. There are three main types: The refracting telescope which uses solely an arrangement of lenses. The reflecting telescope which uses solely an arrangement of mirrors. The catadioptric telescope which uses a combination of mirrors and lenses.

2 Refracting telescopes A refracting or refractor telescope is a dioptric telescope that uses a lens as its objective to form an image. The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used in other devices such as binoculars and long or telephoto camera lenses. Invention: The Early History of the Telescope Refractors were the earliest type of optical telescope. The first practical refracting telescopes appeared in the Netherlands about 1608, and were credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar also known as Jacob Adriaanszoon. Galileo, happening to be in Venice in about the month of May 1609, heard of the invention and constructed a much-improved version of his own based on his understanding of the effects of refraction. Galileo then communicated the details of his invention to the public, and presented the instrument itself to the doge Leonardo Donato, sitting in full council. Galileo may thus claim to have invented the refracting telescope independently, but not until he had heard that others had done so. Refracting telescope designs 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. Diagram: Refracting and Reflecting Telescope

3 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. Galilean telescopes give upright views but suffer from a limited field of view, spherical and chromatic aberration, and poor eye relief. 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 refractors 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 lowdispersion (ED) glass in the objective and produce a very crisp image which 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. Notable refracting telescopes Yerkes Observatory (100cm) Swedish Solar Telescope (100cm) Lick Observatory (91cm) Paris Observatory (83cm + 62cm) Nice Observatory (76cm) Lowell Observatory (24 in) Chabot Space & Science Center (20 in, 8 in)

4 Refracting Telescopes 76cm refractor at Nice Observatory Image of Refracting Telescope from Cincinnati Observatory in 1848

5 Reflecting Telescopes A reflecting telescope (reflector) is an optical telescope which uses a combination of curved or plane (flat) mirrors to reflect light and form an image (catoptric), rather than lenses to refract or bend light to form an image (dioptric). History 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. In 1663 James Gregory published Optica Promota which described the first practical design of a reflector using two concave mirrors. A working example was not built until 10 years later by Robert Hooke. Sir Isaac Newton is credited with constructing the first "practical" reflecting telescope after his own design circa He designed his reflector, which used a concave objective and a smaller "diagonal" mirror, in order to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses. 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 (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. While the Newtonian focus design is still used in amateur astronomy, professionals now tend to use prime focus, Cassegrain focus, and coudé focus designs. The Newtonian Telescope The Newtonian usually has a paraboloid primary mirror but for small apertures of 12 cm 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. Diagram: Newtonian Telescope

6 The Cassegrain telescope 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 "starshaped" 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. Catadioptric Telescopes Diagram: Cassegrain Telescope A catadioptric optical system is one which contains both lenses and mirrors. Catadioptric systems are commonly used in telescopes and in lightweight, long-focus lenses for cameras. Catadioptric telescopes are designs that combine specifically shaped mirrors and lenses to allowing good focal ratios 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. A disadvantage to this design is that the secondary mirror blocks a portion of the light entering the tube. Diagram: Catadoptric Telescope