) is a telescope designed for the sole purpose of astrophotography
. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids
, and comets
Most research telescopes in this class are refractors
although there are many (usually larger) reflecting
designs such as the Ritchey-Chrétien
such as the Schmidt camera
. The main parameters of an Astrograph are the diameter and f-ratio
of the objective
which determine the 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 8-20 inches (20 - 50 cm).
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 (sometimes as larger as 17x17 inches), flat, and distortion-less image at the 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 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 arcsecs/mm
Astrographs used in Astrometry
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.
Astrographs used for stellar classification
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). Both telescopes are individually designed to focus the desired wavelength of light that is combined 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 star’s distances. Sky fields that are photographed twice, decades apart in time, will reveal a nearby stars' 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
, variable stars
, and even unknown planets
. By comparing the pair of images, using a device such as a blink comparator
, astronomers are able to find a 'star' that moved between the two exposures or simply appears in one image only in the case of a nova. 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 know case of an astrograph used in a discovery is Clyde Tombaugh’s discovery of the dwarf planet Pluto in 1930. Tombaugh was given the job of hunting for a suspected “9th planet” which was achieved by systematically photographing the area of the sky around the ecliptic. Tombaugh used Lowell Observatory's 13-inch (3 lens element), f/5.3 refractor astrograph which recorded images on 14x17 inch glass plates.