Citizendia
Your Ad Here

Eight Inch refracting telescope.
Eight Inch refracting telescope.

An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum, for directly viewing a magnified image, making a photograph, or collecting data through electronic image sensors. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. In Geometrical optics, a focus, also called an image point, is the point where Light rays originating from a point on the object converge. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies Magnification is the process of enlarging something only in appearance not in physical size A photograph (often shortened to photo) is an Image created by Light falling on a light-sensitive surface usually Photographic film or an electronic An image sensor is a device that converts an optical image to an electric signal

There are three primary types of optical telescope: Refractors (Dioptrics) which use lenses, Reflectors (Catoptrics) which use mirrors, and Combined Lens-Mirror Systems (Catadioptrics) which use lenses and mirrors in combination (for example the Maksutov telescope and the Schmidt camera). A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image Dioptrics is the study of the Refraction of light especially by lenses. A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging A reflecting telescope (also called a reflector) is an Optical telescope which uses a single or combination of Curved mirrors that reflect Light Catoptrics deals with the phenomena of reflected light and image-forming optical systems using Mirrors From the Greek κατοπτρικός (specular A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. A catadioptric optical system is one which contains both lenses and Mirrors Catadioptric systems are commonly used in Telescopes and in lightweight A catadioptric optical system is one which contains both lenses and Mirrors Catadioptric systems are commonly used in Telescopes and in lightweight The Maksutov is a Catadioptric Telescope design that employs a full diameter meniscus lens (commonly called a "corrector plate" to correct A Schmidt camera, also referred to as the Schmidt telescope, is an astronomical Camera designed to provide wide fields of view with limited

Contents

History

Main article: History of telescopes

The earliest known working telescopes appeared in 1608 and are credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar also known as Jacob Adriaanszoon (although there are claims that Leonard Digges, an English mathematician and surveyor, constructed one in 1570s[1][2]). Hans Lippershey (1570&ndashSeptember 1619 also known as Johann Lippershey or Lipperhey, was a German - Dutch lensmaker. Sacharias Jansen (c 1585 - c 1632 was a Dutch spectacle-maker from Middelburg credited with inventing or contributing advances towards the invention of the Jacob ( Jacobus; sometimes James) Metius (after 1571 Alkmaar - 1628 Alkmaar was a Dutch instrument-maker and optician For the place with the same name in Commewijne District of Suriname, see Alkmaar (Suriname. For Leonard Digges's grandson a minor poet by the same name see Leonard Digges (writer. Galileo Galilei made his own telescope in 1609 after hearing about Hans Lippershey's design, calling it at first a "perspicillum," and then using the terms "telescopium" in Latin and "telescopio" in Italian (from which the English word derives). Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Galileo is generally credited with being the first to use a telescope for astronomical purposes. (The telescope was first used to spot ships. ) Galileo's telescope consisted of a convex object lens and a concave eye lens, which is universally called a Galilean telescope (still used in camera viewfinders and in simple "Galilean binocular" designs). A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image Binocular telescopes, or binoculars (also known as field glasses are two identical or Mirror - symmetrical telescopes mounted side-by-side and Later, Johannes Kepler described the optics of lenses (see his books Astronomiae Pars Optica and Dioptrice), including a new kind of astronomical telescope with two convex lenses (a principle often called the Keplerian Telescope). Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image

How it works

For detailed information on specific designs of reflecting, refracting, and catadioptric telescopes see the main articles on Reflecting telescopes, Refracting telescopes, and Catadioptrics. A reflecting telescope (also called a reflector) is an Optical telescope which uses a single or combination of Curved mirrors that reflect Light A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image A catadioptric optical system is one which contains both lenses and Mirrors Catadioptric systems are commonly used in Telescopes and in lightweight

The basic scheme is that the primary light-gathering element, the objective (1) (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object (4) to a focal plane where it forms a real image (5). An objective in Optics is the lens or Mirror in a Microscope, Telescope, camera or other optical instrument A curved mirror is a Mirror with a curved reflective surface which may be either convex (bulging outward or concave (bulging inward In Optics, a real image is a representation of an actual object (source formed by rays of Light passing through the Image. This image may be recorded, or viewed through an eyepiece (2) which acts like a magnifying glass. For the device for looking through a camera see Viewfinder. An eyepiece, or ocular lens, is a type of lens that is attached magnifying glass (called a hand lens in laboratory contexts is a convex lens which is used to produce a magnified Image of an object The eye (3) sees an inverted magnified virtual image (6) of the object. Magnification is the process of enlarging something only in appearance not in physical size In Optics, a virtual image is an image in which the outgoing rays from a point on the object never actually intersect at a point

Schematic of a Keplerian refracting telescope
Schematic of a Keplerian refracting telescope

Inverted images

Most telescope designs produce an inverted image at the focal plane. A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image These are referred to as inverting telescopes. In astronomical telescopes the inverted view is normally not corrected, since it does not affect how the telescope is used. In terrestrial telescopes such as Spotting scopes, monoculars, and binoculars, prisms (e. A spotting scope is a portable Telescope, optimized for the observation of terrestrial objects A monocular is a modified Refracting telescope used to Magnify the images of distant objects by passing light through a series of lenses and prisms Binocular telescopes, or binoculars (also known as field glasses are two identical or Mirror - symmetrical telescopes mounted side-by-side and g. Porro prisms) or a relay lens between objective and eyepiece are used to invert the image once more to a correct orientation. In Optics, a Porro prism, named for its inventor Ignazio Porro, is a type of reflection prism used in optical instruments to alter the orientation There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector. A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image The Gregorian telescope is a type of Reflecting telescope designed by Scottish Mathematician and Astronomer, James Gregory in the These are referred to as erecting telescopes.

Design variants

Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design (Newtonian telescope, Cassegrain reflector or similar types), or may simply be used to place the eyepiece or detector at a more convenient position. The Newtonian telescope is a type of Reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727 using a parabolic primary mirror The Cassegrain reflector is a combination of two coaxial reflectors used in Cassegrain telescopes and radio antennas First developed in 1672 by Laurent Cassegrain Telescope designs may also use specially designed additional lenses or mirrors to improved image quality over a larger field of view.

Angular resolution

Ignoring blurring of the image by turbulence in the atmosphere (atmospheric seeing) and optical imperfections of the telescope, the angular resolution of an optical telescope is determined by the width of the objective, termed its "aperture" (the primary mirror, or lens. Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by Turbulence in the Earth's atmosphere Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera ) The Rayleigh criterion for the resolution limit αR (in radians) is given by

αR = 1. Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera The radian is a unit of plane Angle, equal to 180/ π degrees, or about 57 22λ / D,

where λ is the wavelength and D is the aperture. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. For visible light (λ = 550 nm), this equation can be rewritten:

αR = 138 / D.

Here, αR denotes the resolution limit in arcseconds and D is in millimeters. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. In the ideal case, the two components double stars can be split even if separated by slightly less than αR. Double Star is a Science fiction Novel by Robert A Heinlein, first serialized in Astounding Science Fiction This is taken into account by the Dawes limit

αD = 116 / D.

Essentially; the larger the aperture, the better the angular resolution

It should be noted that the resolution is NOT given by the maximum magnification (or "power") of a telescope. Magnification is the process of enlarging something only in appearance not in physical size Telescopes marketed by giving high values of the maximum power often deliver poor images.

For large ground-based telescopes, the resolution is limited by atmospheric seeing. Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by Turbulence in the Earth's atmosphere This limit can be overcome by placing the telescopes above the atmosphere, e. g. , on the summits of high mountains, on balloon and high-flying airplanes, or in space. A space observatory is any instrument in Outer space which is used for observation of distant planets galaxies and other outer space objects Resolution limits can also be overcome by adaptive optics, speckle imaging or lucky imaging for ground-based telescopes. Adaptive optics (AO is a Technology used to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion Speckle imaging (also known Lucky imaging (also called lucky exposures) is one form of Speckle imaging used for Astronomical photography.

Recently, it has become practical to perform aperture synthesis with arrays of optical telescopes. Very high resolution images can be obtained with groups of widely-spaced smaller telescopes, linked together by carefully controlled optical paths, but these interferometers can only be used for imaging bright objects such as stars or measuring the bright cores of active galaxies. Current Performance of Ground-Based Interferometers Here is a list of currently existing astronomical optical interferometers (i An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic Example images of starspots on Betelgeuse can be seen here. Betelgeuse (ˈbiːtəldʒuːz or /ˈbɛtəldʒuːz/ ( α Ori α Orionis Alpha Orionis is a Semiregular variable star located 640 Light-years away from

Focal length and f-ratio

The focal length determines how wide an angle the telescope can view with a given eyepiece or size of a CCD detector or photographic plate. For the device for looking through a camera see Viewfinder. An eyepiece, or ocular lens, is a type of lens that is attached A charge-coupled device ( CCD) is an analog Shift register, that enables the transportation of analog signals (electric charges through successive stages (capacitors Photographic plates preceded Photographic film as a mean of photography The f-ratio (or focal ratio, or f-number) of a telescope is the ratio between the focal length and the aperture (i. The focal length of an optical system is a measure of how strongly it converges (focuses or diverges (diffuses Light. e. , diameter) of the objective. Thus, for a given aperture (light-gathering power), low f-ratios indicate wide fields of view. Wide-field telescopes (such as astrographs) are used to track satellites and asteroids, for cosmic-ray research, and for surveys of the sky. An Astrograph ( astrographic camera) is a telescope designed for the sole purpose of Astrophotography. This article is about artificial satellites For natural satellites also known as moons see Natural satellite. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on It is more difficult to reduce optical aberrations in telescopes with low f-ratio than in telescopes with larger f-ratio. Aberrations are departures of the performance of an optical system from the predictions of Paraxial optics.

Light-gathering power

The light-gathering power of an optical telescope is directly related to the diameter (or aperture) of the objective lens or mirror. Geometry, a diameter of a Circle is any straight Line segment that passes through the center of the circle and whose Endpoints are on the Note that the area of a circle is proportional to the square of the radius. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.

Imperfect images

No telescope can form a perfect image. Even if a reflecting telescope could have a perfect mirror, or a refracting telescope could have a perfect lens, the effects of aperture diffraction could still not be escaped. In reality, perfect mirrors and perfect lenses do not exist, so image aberrations in addition to aperture diffraction must be taken into account. Aberrations are departures of the performance of an optical system from the predictions of Paraxial optics. Image aberrations can be broken down into two main classes, monochromatic, and polychromatic. In 1857, Philipp Ludwig von Seidel (1821–1896) decomposed the first order monochromatic aberrations into five constituent aberrations. Philipp Ludwig von Seidel ( October 24, 1821 Zweibrücken, Germany – August 13, 1896, Munich) was a German They are now commonly referred to as the five Seidel Aberrations.

The five Seidel aberrations

Main article: Optical aberration
Spherical aberration 
The difference in focal length between paraxial rays and marginal rays, proportional to the square of the aperture. Aberrations are departures of the performance of an optical system from the predictions of Paraxial optics. spherical-aberration-diskjpg|thumb|300 px|left|A Point source as imaged by a system with negative (top zero (center and positive (bottom spherical aberration
Coma 
A most objectionable defect by which points are imaged as comet-like asymmetrical patches of light with tails, which makes measurement very imprecise. In Optics (especially Telescopes, the coma (aka comatic aberration) in an optical system refers to aberration inherent to certain optical Its magnitude is usually deduced from the optical sine theorem. In Optics, the optical sine theorem states that the products of the index height and sine of the slope angle of a ray in object space and its corresponding ray in image space
Astigmatism 
The image of a point forms focal lines at the sagittal and tangental foci and in between (in the absence of coma) an elliptical shape. See also Aberration in optical systems, Astigmatism (eye An Optical system with astigmatism is one where rays that propagate
Curvature of Field
The Petzval curvature means that the image instead of lying in a plane actually lies on a curved surface which is described as hollow or round. Joseph Petzval (Josef Maximilian Petzval Petzvál József Miksa Jozef Maximilián Petzval ( January 6, 1807 &ndash September 19, 1891) was This causes problems when a flat imaging device is used e. g. a photographic plate or CCD image sensor.
Distortion
Either barrel or pincushion, a radial distortion which must be corrected for if multiple images are to be combined (similar to stitching multiple photos into a panoramic photo). Panoramic photography is a format of Photography that aims to create images with exceptionally wide fields of view, but has also come to refer to any photograph that

They are always listed in the above order since this expresses their interdependence as first order aberrations via moves of the exit/entrance pupils. The first Seidel aberration, Spherical Aberration, is independent of the position of the exit pupil (as it is the same for axial and extra-axial pencils). The second, coma, changes as a function of pupil distance and spherical aberration, hence the well-known result that it is impossible to correct the coma in a lens free of spherical aberration by simply moving the pupil. Similar dependencies affect the remaining aberrations in the list.

The chromatic aberrations

Longitudinal Chromatic Aberration
As with spherical aberration this is the same for axial and oblique pencils.
Transverse Chromatic Aberration (Chromatic Aberration of Magnification)

Optical Research telescopes

Harlan J. Smith Telescope at McDonald Observatory, Texas
Harlan J. Smith Telescope at McDonald Observatory, Texas

Nearly all large research-grade astronomical telescopes are reflectors. The Harlan J Smith Telescope is a 27m (107-in telescope located at the McDonald Observatory ( The McDonald Observatory is located near the unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. Some reasons are:

Most large research telescopes can operate as either a Cassegrain telescope (longer focal length, and a narrower field with higher magnification) or a Newtonian telescope (brighter field). The Cassegrain reflector is a combination of two coaxial reflectors used in Cassegrain telescopes and radio antennas First developed in 1672 by Laurent Cassegrain The Newtonian telescope is a type of Reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727 using a parabolic primary mirror They have a pierced primary mirror, a Newtonian focus, and a spider to mount a variety of replaceable secondary mirrors.

A new era of telescope making was inaugurated by the Multiple Mirror Telescope (MMT), with a mirror composed of six segments synthesizing a mirror of 4. The MMT Observatory (MMTO is an Astronomical observatory on the site of Fred Lawrence Whipple Observatory (IAU 5 meters diameter. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International This has now been replaced by a single 6. 5 m mirror. Its example was followed by the Keck telescopes with 10 m segmented mirrors. The W M Keck Observatory is a two-telescope Astronomical observatory at the 4145 meter (13600 ft summit of Mauna Kea in Hawai'i.

The largest current ground-based telescopes have a primary mirror of between 6 and 11 meters in diameter. A primary mirror (or primary) is the principal Light -gathering surface of a reflective Telescope. In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see active optics). Active optics is a relatively new Technology for Reflecting telescopes developed in the 1980s which has more recently enabled the construction of a generation of telescopes This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters.

Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed. Many are robotic telescopes, computer controlled over the internet (see e. A robotic telescope is an astronomical Telescope and detector system that makes Observations without the intervention of a Human. g. the Liverpool Telescope and the Faulkes Telescope North and South), allowing automated follow-up of astronomical events. The Liverpool Telescope is a 2 m fully Robotic telescope, meaning that it can be remotely operated and also run without human intervention once given a list of observations The Faulkes Telescope North is a clone of the Liverpool Telescope, and is located at Haleakala Observatory in the U The Faulkes Telescope South (which was cloned to build the Liverpool Telescope) and is located at Siding Spring Observatory.

Initially the detector used in telescopes was the human eye. A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain Later, the sensitized photographic plate took its place, and the spectrograph was introduced, allowing the gathering of spectral information. Photographic plates preceded Photographic film as a mean of photography A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used After the photographic plate, successive generations of electronic detectors, such as the charge-coupled device (CCDs), have been perfected, each with more sensitivity and resolution, and often with a wider wavelength coverage. A charge-coupled device ( CCD) is an analog Shift register, that enables the transportation of analog signals (electric charges through successive stages (capacitors

Current research telescopes have several instruments to choose from such as:

The phenomenon of optical diffraction sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the Airy disc, which limits how close two such discs can be placed. Diffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle The Airy disk (or Airy disc) is a phenomenon in Optics. Owing to the wave nature of light, light passing through an Aperture is diffracted This absolute limit is called the diffraction limit (or sometimes the Rayleigh criterion, Dawes limit or Sparrow's resolution limit). The resolution of an optical imaging system like a Microscope or Telescope or Camera can be limited by multiple factors like imperfections in the lenses or misalignment Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera Dawes' limit is a formula to express the maximum Resolving power of a Microscope or Telescope. This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue light) and on the diameter of the telescope mirror. Geometry, a diameter of a Circle is any straight Line segment that passes through the center of the circle and whose Endpoints are on the This means that a telescope with a certain mirror diameter can theoretically resolve up to a certain limit at a certain wavelength. For conventional telescopes on Earth, the diffraction limit is not relevant for telescopes bigger than about 10 cm. Instead, the seeing, or blur caused by the atmosphere, sets the resolution limit. Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by Turbulence in the Earth's atmosphere But in space, or if adaptive optics are used, then reaching the diffraction limit is sometimes possible. Adaptive optics (AO is a Technology used to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion At this point, if greater resolution is needed at that wavelength, a wider mirror has to be built or aperture synthesis performed using an array of nearby telescopes.

In recent years, a number of technologies to overcome the distortions caused by atmosphere on ground-based telescopes have been developed, with good results. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five See adaptive optics, speckle imaging and optical interferometry. Adaptive optics (AO is a Technology used to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion Speckle imaging (also known Optical interferometry combines two or more light waves in an optical instrument in such a way that Interference occurs between them

Famous optical telescopes

The Hubble Space Telescope orbits above Earth.
The Hubble Space Telescope orbits above Earth. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into

Notes

  1. ^ chocky.demon.co.uk - Did the reflecting telescope have English origins?
  2. ^ The New Werner Twentieth Century Edition of the Encyclopaedia Britannica, 1907 p147

See also

External links

Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study The field of amateur telescope making is considered an offshoot of the Amateur astronomy community A dipleidoscope is an instrument used to determine true Noon. Several hundred more observatories (many optical are listed here A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation.
© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
 Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic