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 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 (for example, 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
Read more about this topic: Astrograph
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