An astrograph (or astrographic camera) is a telescope designed for the sole purpose of astrophotography. Astrographs are mostly used in wide-field astronomical surveys of the sky and for detection of objects such as asteroids, meteors, and comets. Improvements in photography 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 photographic plate or sometimes on photographic film. Many observatories of this period used an astrograph, beside instruments like the transit telescope, great refractors, and chronometers, or instruments for observing the Sun. Astrographs were often used to make surveys of the night sky, and one of the famous projects was Carte du Ciel. Discoveries using an astrograph include then-planet Pluto. Rather than looking through the telescope, it was discovered by using a blink comparator 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. 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 . 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 distortionless 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 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