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Encyclopedia > Refracting telescope
Image of a refracting telescope from the Cincinnati Observatory in 1848
Image of a refracting telescope from the Cincinnati Observatory in 1848

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 spy glasses and astronomical telescopes but is also used in other devices such as binoculars and long or telephoto camera lenses. Image File history File linksMetadata Download high-resolution version (1218x1800, 365 KB) THE 2d TELESCOPE IN SIZE IN THE UNITED STATES AT THE CINCNNATI OBSERVATORY. Cover and inside illustration from: Smiths Illustrated Astronomy, Designed for the use of the Public Or Common Schools in the United States. ... Image File history File linksMetadata Download high-resolution version (1218x1800, 365 KB) THE 2d TELESCOPE IN SIZE IN THE UNITED STATES AT THE CINCNNATI OBSERVATORY. Cover and inside illustration from: Smiths Illustrated Astronomy, Designed for the use of the Public Or Common Schools in the United States. ... Categories: Wikipedia cleanup | Telescopes | Astronomy stubs ... This article does not cite any references or sources. ... A lens. ... Several objective lenses on a microscope. ... Astronomy, which etymologically means law of the stars, (from Greek: αστρονομία = άστρον + νόμος) is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. ... Porro-prism binoculars with central focusing Binocular telescopes, or binoculars, (also known as field glasses) are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. ... In photography and cinematography, a telephoto lens is a lens whose focal length is significantly longer than the focal length of a normal lens. ...

Contents

Invention

Main article: History of telescopes

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. This article or section is not written in the formal tone expected of an encyclopedia article. ... Eight Inch refracting telescope. ... Events March 18 - Sissinios formally crowned Emperor of Ethiopia May 14 - Protestant Union founded in Auhausen. ... Hans Lippershey (1570–September 1619) was a Dutch lensmaker. ... This article or section does not adequately cite its references or sources. ... This is about the city in the Netherlands. ... Jacob (or Jacobus) Metius (died between 1624 and 1631), Dutch instrument-maker and optician. ... Accijnstoren Cheese market Canal and bridge Weighing house Alkmaar (West Frisian: Alkmare) is a municipality and a city in the Netherlands, in the province of Noord Holland. ... Galileo Galilei (15 February 1564 – 8 January 1642) was an Italian physicist, mathematician, astronomer, and philosopher who is closely associated with the scientific revolution. ... Venice (Italian: Venezia, Venetian: Venezsia, Latin: Venetia) is a city in northern Italy, the capital of region Veneto, and has a population of 271,251 (census estimate January 1, 2004). ... // Events April 4 – King of Spain signs an edit of expulsion of all moriscos from Spain April 9 – Spain recognizes Dutch independence May 23 - Official ratification of the Second Charter of Virginia. ... Grand Procession of the Doge, 16th century For about a thousand years, the chief magistrate and leader of the Most Serene Republic of Venice was styled the Doge, a rare but not unique Italian title derived from the Latin Dux, as the major Italian parallel Duce and the English Duke. ...


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. Look up convex in Wiktionary, the free dictionary. ... Several objective lenses on a microscope. ... A collection of different types of eyepieces. ... The straw seems to be broken, due to refraction of light as it emerges into the air. ... This article does not cite any references or sources. ... A lens. ... Parallel is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. ... Child – 5:16 All I Need – 3:55 Drifting – 6:43 Hold On – 4:40 Open Me – 3:35 Beautiful – 5:44 Look In – 4:14 Without You – 4:55 Live It – 7:23 Dont Walk Away – 3:04 Lead Me On – 5:34 Rest – 5:06 Child [Piano... The focal plane of a lens is a plane that is perpendicular to the axis of the lens and passes through its focus. ... Brightness is an attribute of visual perception in which a source appears to emit a given amount of light. ... Clarity is the property of being clear or transparent. ... You may be looking for one of the following: Dimensions: length, width, height Clothing measurements such as shoe size or dress size Geometry Measurement Gelatinous or glutinous substance made from glue, wax, clay or similar Or the following command-line Unix tool: Size (Unix) This is a disambiguation page: a...


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). Johannes Kepler (December 27, 1571 – November 15, 1630) was a German mathematician, astronomer and astrologer, and a key figure in the 17th century astronomical revolution. ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ... Johannes Hevelius Johannes Hevelius (Latin), also called Johann Hewelke, Johannes Höwelcke or Johannes Hewel (in German), or Jan Heweliusz (in Polish), (born January 28, 1611 – died January 28, 1687), was a councillor and mayor in Danzig (Gdańsk). ... The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror. ... External, internal, and depth micrometers A micrometer is a widely used device in mechanical engineering for precisely measuring thickness of blocks, outer and inner diameters of shafts and depths of slots. ...


Achromatic refractors

Main article: 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. For an achromatic doublet, visible wavelengths have approximately the same focal length. ... Diagram of an achromatic lens (doublet). ... Chester Moore Hall (1703–1771) was a British lawyer who produced the first achromatic lenses in 1729 or 1733 (accounts differ). ... John Dollond (June 10, 1706 - November 30, 1761) was an English optician. ... Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. ... Focal plane Longitudinal sections In optics, spherical aberration is an image imperfection that occurs due to the increased refraction of light rays that occurs when rays strike a lens or mirror near its edge, in comparison with those that strike nearer the center. ... Glass can be made transparent and flat, or into other shapes and colors as shown in this sphere from the Verrerie of Brehat in Brittany. ... Dispersion of a light beam in a prism. ... Crown glass is a type of optical glass used in lenses. ... Flint glass is an optical glass that has relatively high refractive index and low Abbe number. ... The wavelength is the distance between repeating units of a wave pattern. ...


Apochromatic refractors

Main article: Apochromat

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. An apochromat, or apo lens, is a photographic or other lens that has a high degree of color correction. ... Fluorite (also called fluor-spar) is a mineral composed of calcium fluoride, CaF2. ...


Technical considerations

Refractors have been criticized for their relatively high-degree of residual chromatic and spherical aberration. This affects shorter focal lengths more than longer ones. A 4" f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 4" f/16 will have little color fringing. Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. ... Focal plane Longitudinal sections In optics, spherical aberration is an image imperfection that occurs due to the increased refraction of light rays that occurs when rays strike a lens or mirror near its edge, in comparison with those that strike nearer the center. ... The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror. ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ...


In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass. There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished by using reflecting telescopes, that can be made in far larger apertures. Lens sag is a problem that sometimes afflicts very large reflecting telescopes. ... Gravity is a force of attraction that acts between bodies that have mass. ... Glass can be made transparent and flat, or into other shapes and colors as shown in this sphere from the Verrerie of Brehat in Brittany. ... Look up air in Wiktionary, the free dictionary. ... Soap bubbles Bubble may refer to: Soap bubble, spherical liquid film, also possibly of bubble gum Cavitation, pocket of air caught in a liquid Bubble (economics), where speculation causes prices to rise to unsustainable levels a (normally) transparent dome Light bulb, in theater lighting terminology [1] in poker tournaments, the... A substance or object that is opaque is neither transparent nor translucent. ... The wavelength is the distance between repeating units of a wave pattern. ... The visible spectrum is the portion of the optical spectrum (light or electromagnetic spectrum) that is visible to the human eye. ...


Notable refracting telescopes

See also


 
 

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