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An astrograph (or astrographic camera) is a
telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

telescope
designed for the sole purpose of
astrophotography Astrophotography, also known as astronomical imaging, is of s, s, and areas of the . The first photograph of an astronomical object (the ) was taken in 1840, but it was not until the late 19th century that advances in technology allowed for deta ...

astrophotography
. Astrographs are mostly used in wide-field
astronomical survey An astronomical survey is a general map A map is a symbol A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, Object (philosophy), object, or wikt:relationship, relationship. Symbols a ...
s of the
sky The sky is the panorama obtained from observing the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...

sky
and for detection of objects such as
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s,
meteor A meteoroid () is a small rocky or metallic body in outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth Earth is the third planet from the Sun and the only astronomical object known to h ...

meteor
s, and
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s.
Improvements in photography
Improvements in photography
in the middle 19th century led to designs dedicated to astrophotography, and they were also popular in the
20th century The 20th (twentieth) century began on January 1, 1901 ( MCMI), and ended on December 31, 2000 ( MM). It was the tenth and final century of the 2nd millennium. The 20th century was dominated by significant events that defined the era: Spanish ...
. As in other photography, chemicals were used that respond to light, recorded on a glass
photographic plate photographic plates, 1880 Image:Femme-au-chien neg.jpg, Negative plate Photographic plates preceded photographic film Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin photographic emulsion, emu ...
or sometimes on
photographic film Photographic film is a strip or sheet of transparent film baseA film base is a transparent substrate which acts as a support medium for the photosensitive emulsion An emulsion is a mixture of two or more liquids that are normally Miscibil ...
. Many observatories of this period used an astrograph, beside instruments like the
transit telescopeIn astronomy, a transit instrument is a small telescope with extremely precisely graduated telescope mount, mount used for the precise observation of star positions. They were previously widely used in astronomical observatory, astronomical observat ...
,
great refractor Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy defines an era in modern telescopy in the 19th ...
s, and chronometers, or instruments for observing the Sun. Astrographs were often used to make surveys of the
night sky The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when ...

night sky
, and one of the famous projects was
Carte du Ciel The Carte du Ciel (literally, 'Map of the Sky') and the Astrographic Catalogue (or Astrographic Chart) were two distinct but connected components of a massive international astronomical project, initiated in the late 19th century, to catalogue and ...
. Discoveries using an astrograph include then-planet
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
. Rather than looking through the telescope, it was discovered by using a
blink comparator Blinking is a bodily function; it is a semi-autonomic rapid closing of the eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outsi ...
with images taken by an astrograph. By the late 20th century, electronic detectors became more common with the data being stored electronically.


Design

Most research telescopes in this class are refractors, although there are many (usually larger) reflecting designs such as the Ritchey-Chrétien and catadioptrics such as the
Schmidt camera Optical ray paths inside a Schmidt camera A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope A telescope is an optical instrument using lenses, curved mirrors, or a combination of both ...
. The main parameters of an Astrograph are the diameter and
f-ratio
f-ratio
of the
objective Objective may refer to: * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pronoun that is used as a grammatical object * Objective Productions, a Briti ...
, which determine the
field of view The field of view (FoV) is the extent of the observable world that is at any given moment. In the case of s or sensors it is a through which a detector is sensitive to . Humans and animals In the context of human and primate vision, th ...

field of view
and image scale on the photographic plate or CCD detector. The objective of an astrograph is usually not very large, on the order of . The shape of the focal plane is often designed to work in conjunction with a specific shaped photographic plate or CCD detector. The objective is designed to produce a particularly large (for example, ), flat, and distortion-less image at the
focal plane In Gaussian optics, the cardinal points consist of three pairs of Point (geometry), points located on the optical axis of a Rotational symmetry, rotationally symmetric, focal, optical system. These are the Focus (optics), focal points, the princi ...

focal plane
. They may even be designed to focus certain wavelengths of light to match the type of film they are designed to use. (Early astrographs were corrected to work in blue wavelengths to match photographic emulsions of the time.) Wide-angle astrographs with short f-ratios are used for photographing a huge area of sky. Astrographs with higher f-ratios are used in more precise measurements. Many observatories of the world are equipped with the so-called normal astrographs with an aperture of around and a focal length of . The purpose of a "normal astrograph" is to create images where the scale of the image at the focal plane is a standard of approximately 60 arcsecs/mm.


Applications


Astrometry

Astrographs used in
astrometry in the optical wavelength range to determine precise positions of stars. ''Courtesy NASA/JPL-Caltech'' Astrometry is the branch of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of ...
record images that are then used to "map" the positions of objects over a large area of the sky. These maps are then published in catalogs to be used in further study or to serve as reference points for deep-space imaging.


Stellar classification

Astrographs used for
stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectr ...
sometimes consist of two identical telescopes on the same mount (a double astrograph). Each sky field can be simultaneously photographed in two colors (usually blue and yellow). Each telescope may have individually designed
non-achromatic objectiveA non-achromatic objective is an objective lens which is not corrected for chromatic aberration. In Optical telescope, telescopes they can a be pre-18th century simple single element objective lenses which were used before the invention of Doublet (l ...
s to focus the desired wavelength of light which is paired with the respective color-sensitive (black-and-white) photographic plate. In other cases a single telescope is used to make two exposures of the same part of the sky with different filters and color sensitive film used on each exposure. Two-color photography lets astronomers measure the color, as well as the brightness (magnitude), of each star imaged. Colors tell the star's "temperature”. Knowing the color type and magnitudes lets astronomers determine the distance of a star. Sky fields that are photographed twice, decades apart in time, will reveal a nearby star's
proper motion Proper motion is the astrometry, astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the m ...

proper motion
when measured against the background of distant stars or galaxies.


Discovery of astronomical objects

By taking two exposures of the same section of the sky days or weeks apart, it is possible to find objects such as
asteroids An asteroid is a minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a ...

asteroids
,
meteor A meteoroid () is a small rocky or metallic body in outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth Earth is the third planet from the Sun and the only astronomical object known to h ...

meteor
s,
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s,
variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either: ...
s,
nova A nova (plural novae or novas) is a transient astronomical eventA transient astronomical event, often shortened by astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field out ...

nova
e, and even unknown
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s. By comparing the pair of images, using a device such as a
blink comparator Blinking is a bodily function; it is a semi-autonomic rapid closing of the eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outsi ...
, astronomers are able to find objects that moved or changed brightness between the two exposures or simply appear in one image only, as in the case of a nova or meteor. Sometimes objects can even be found in one exposure since a fast moving object will appear as a "line" in a long exposure. One well-known case of an astrograph used in a discovery is
Clyde Tombaugh Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was co ...
’s discovery of the
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
in 1930. Tombaugh was given the job of hunting for a suspected "9th planet" to be achieved by systematically photographing the area of the sky around the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the ...

ecliptic
. Tombaugh used
Lowell Observatory Lowell Observatory is an astronomical observatory in Flagstaff, Arizona Flagstaff ( ; nv, Kinłání Dookʼoʼoosłííd Biyaagi, ) is a city in, and the county seat of, Coconino County, Arizona, Coconino County in northern Arizona, in the s ...

Lowell Observatory
's (3 lens element), f/5.3 refractor astrograph, which recorded images on glass plates.


Use in amateur astrophotography

In the
amateur astronomy Image:Astronomy Amateur 3 V2.jpg, 250px, Amateur astronomers watch the night sky during the Perseids, Perseid meteor shower. Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the Naked e ...
field, many types of commercial and amateur built telescopes are designed for
astrophotography Astrophotography, also known as astronomical imaging, is of s, s, and areas of the . The first photograph of an astronomical object (the ) was taken in 1840, but it was not until the late 19th century that advances in technology allowed for deta ...

astrophotography
and labeled "astrographs". Optical designs of amateur astrographs vary widely but include
apochromatic An apochromat, or apochromatic lens (apo), is a Photographic lens, photographic or other Lens (optics), lens that has better correction of chromatic aberration, chromatic and spherical aberration than the much more common achromat lenses. Explanat ...
refractors, variations of Cassegrain reflectors, and
Newtonian reflector The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was ...

Newtonian reflector
s. Most optical designs do not produce large, flat, and well-corrected imaging fields and therefore require some type of optical correction by way of field flatteners or
coma A coma is a deep state of prolonged unconsciousness Unconsciousness is a state which occurs when the ability to maintain an consciousness, awareness of self and environment is lost. It involves a complete, or near-complete, lack of responsive ...
correctors. Amateur astrographs typically have purpose-built focusers, are constructed of thermally stable materials like carbon fiber, and are put on heavy duty mounts to facilitate accurate tracking of deep sky objects for long periods of time.


See also

*
BOOTES
BOOTES
*
List of telescope types The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...


References


External links


The Double Astrograph of the Yale Southern Observatory
{{Commons, Astrograph Telescopes Cameras by type Optical devices Astronomical instruments ja:天体写真儀