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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
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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.
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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, 17 by 17 inches (430 mm × 430 mm)), flat, and distortionless image at the
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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.
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with an aperture of around 13 inches (330 mm) and a focal length of 11 feet (3.4 m). 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
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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
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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
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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.
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in the middle 19th century led to designs dedicated to astrophotography, and they were also popular in the 20th century. As in other photography, chemicals were used that respond to light, recorded on a glass
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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
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Tombaugh, Clyde (5 April 1995). The
Struggles to Find the Ninth Planet. NASA/JPL Outer Planets/Solar Probe Project, 5 April 1995. Retrieved from
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detector. The objective of an astrograph is usually not very large, on the order of 20 to 50 cm (8 to 20 in).
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By taking two exposures of the same section of the sky days or weeks apart, it is possible to find objects such as
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A double astrograph consisting of two 6-inch (150 mm) astrographs and a central guide scope on display at
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By the late 20th century, electronic detectors became more common with the data being stored electronically.
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and labeled "astrographs". Optical designs of amateur astrographs vary widely but include
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http://ircamera.as.arizona.edu/NatSci102/NatSci102/text/ext9thplanet.htm
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Construction of the Tycho
Reference Catalogue – 2 Source Catalogues
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when measured against the background of distant stars or galaxies.
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One well-known case of an astrograph used in a discovery is
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astrograph, specifically designed for astrophotography.
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The Double
Astrograph of the Yale Southern Observatory
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134:A 13-inch (330 mm), f/5.3 astrograph at
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118:Learn how and when to remove this message
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