Optical telescope

telescopetelescopesopticaloptical telescopestelescopiclight-gathering powerlight-gatheringmedium8m telescopesan optical telescope
An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.wikipedia
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Binoculars

binocularfield glassesfield glass
People use telescopes and binoculars for activities such as observational astronomy, ornithology, pilotage and reconnaissance, and watching sports or performance arts. In terrestrial telescopes such as spotting scopes, monoculars and binoculars, prisms (e.g., Porro prisms) or a relay lens between objective and eyepiece are used to correct the image orientation.
Binoculars or field glasses are two telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects.

Eyepiece

Huygens eyepieceeyepiece lensPlossl
Galileo's telescope used a convex objective lens and a concave eye lens, a design is now called a Galilean telescope. This image may be recorded or viewed through an eyepiece (2), which acts like a magnifying glass.
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes.

Telescope

telescopestelescopicspyglass
An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.

Refracting telescope

refractorrefractor telescopeGalilean telescope
Galileo's telescope used a convex objective lens and a concave eye lens, a design is now called a Galilean telescope. It is in the Netherlands in 1608 where the first documents describing a refracting optical telescope surfaced in the form of a patent filed by spectacle maker Hans Lippershey, followed a few weeks later by claims by Jacob Metius, and a third unknown applicant, that they also knew of this "art".
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope).

Catadioptric system

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A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which uses both a lens (corrector plate) and mirror as primary optical elements, mainly used for wide field imaging without spherical aberration.
Catadioptric combinations are used in focusing systems such as searchlights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses.

Dioptrics

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Telescopes that create their image with an objective that is a convex lens (refractors) are said to be "dioptric" telescopes.

Observational astronomy

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People use telescopes and binoculars for activities such as observational astronomy, ornithology, pilotage and reconnaissance, and watching sports or performance arts.
For much of the history of observational astronomy, almost all observation was performed in the visual spectrum with optical telescopes.

Schmidt camera

Schmidt telescopeSchmidtBaker-Nunn camera
A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which uses both a lens (corrector plate) and mirror as primary optical elements, mainly used for wide field imaging without spherical aberration.
A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations.

Jacob Metius

It is in the Netherlands in 1608 where the first documents describing a refracting optical telescope surfaced in the form of a patent filed by spectacle maker Hans Lippershey, followed a few weeks later by claims by Jacob Metius, and a third unknown applicant, that they also knew of this "art".
He is primarily known for being the brother of the geometer and astronomer Adriaan Metius and for the patent application he made for an optical telescope in October 1608, a few weeks after Hans Lippershey submitted a patent for the same device.

Astronomical seeing

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The late 20th century has seen the development of adaptive optics and space telescopes to overcome the problems of astronomical seeing.
The FWHM of the point spread function (loosely called seeing disc diameter or "seeing") is the best possible angular resolution that can be achieved by an optical telescope in a long-exposure image, and corresponds to the FWHM of the fuzzy blob seen when observing a point-like source (such as a star) through the atmosphere.

Angular resolution

resolutionRayleigh criterionresolved
It is analogous to angular resolution, but differs in definition: instead of separation ability between point-light sources it refers to the physical area that can be resolved.
Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolution.

Magnification

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An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.
The angular magnification of an optical telescope is given by

Double star

visual companiondouble starsoptical double
In the ideal case, the two components of a double star system can be discerned even if separated by slightly less than \alpha_R.
In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.

Optics

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The focal length of an optical system is a measure of how strongly the system converges or diverges light.
Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics.

Spotting scope

hunter's telescopespotting scopes
In terrestrial telescopes such as spotting scopes, monoculars and binoculars, prisms (e.g., Porro prisms) or a relay lens between objective and eyepiece are used to correct the image orientation.
The light-gathering power and resolution of a spotting scope is determined by the diameter of the objective lens, typically between 50 and 80 mm.

Astrophotography

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A fast system is often desired for practical purposes in astrophotography with the purpose of gathering more photons in a given time period than a slower system, allowing time lapsed photography to process the result faster.
Photography revolutionized the field of professional astronomical research, with longtime exposures recording hundreds of thousands of new stars and nebulae that were invisible to the human eye, leading to specialized and ever larger optical telescopes that were essentially big cameras designed to record light using photographic plates.

Porro prism

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In terrestrial telescopes such as spotting scopes, monoculars and binoculars, prisms (e.g., Porro prisms) or a relay lens between objective and eyepiece are used to correct the image orientation.
Double Porro prism systems are used in small optical telescopes to re-orient an inverted image (an arrangement is known as an image erection system), and especially in many binoculars where they both erect the image and provide a longer, folded distance between the objective lenses and the eyepieces.

Exit pupil

exitRamsden circle/disc/eyepointray cone angle and brightness
The increase in brightness with reduced magnification has a limit related to something called the exit pupil.
In a telescope or compound microscope, this image is the image of the objective element(s) as produced by the eyepiece.

Cassegrain reflector

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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 Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture.

Lens

lensesconvex lensoptical lens
The lens and the properties of refracting and reflecting light had been known since antiquity, and theory on how they worked was developed by ancient Greek philosophers, preserved and expanded on in the medieval Islamic world, and had reached a significantly advanced state by the time of the telescope's invention in early modern Europe.
Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors.

Galaxy

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Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.

List of largest optical reflecting telescopes

largest optical telescopeslargest optical telescopelargest telescope in the world
This list of the largest optical reflecting telescopes with objective diameters of 3.0 m or greater is sorted by aperture, which is one limit on the light-gathering power and resolution of a reflecting telescope's optical assembly.

List of largest optical telescopes historically

largestworld's largest single-aperture optical telescope
The following is a list of largest single mount optical telescopes sorted by total objective diameter (aperture), including segmented and multi-mirror configurations.

Magnifying glass

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This image may be recorded or viewed through an eyepiece (2), which acts like a magnifying glass.