An astrograph is any type of telescope that has been designed specifically for capturing images of astronomical objects, a practice known as astrophotography. These telescopes are also often known as astronomical cameras. The telescope and camera may be integrated in an astrograph, or they may be separate units, depending on the needed applications. There are both professional and amateur applications and configurations. Astrophotography has grown and evolved in tandem with telescopes and cameras since its successful use in 1840 by John William Draper to capture a daguerreotype of the moon.
The development of the charged coupled device (CCD) cameras and the vast improvements in the technology controlling pixels in the 1990s allowed astronomical cameras to develop dramatically. Large telescopes in research facilities worldwide have been equipped with digital imaging devices to improve their images. Prior to CCD technology being widely accepted, researchers collected the images on photographic plates, which require a great deal of preparation for use.
Discovery and classification of celestial objects as well as mapping deep space and large fields of the sky are primary applications of an astrograph. These applications typically use an integrated design and tend to be refractor telescopes that work in conjunction with a CCD detector or camera. Larger projects, such as the large binocular telescope in Arizona in the U.S., often use a reflector telescope. A few of the older astrograph projects still utilize photographic plates instead of cameras to capture images.
Amateur and professional astrophotographers typically use a coupled design, adding a digital single lens reflex (DSLR) or CCD camera to a telescope. DSLR cameras have the advantage of being dual function, meaning they can be used for both celestial and terrestrial photography but are difficult to cool and require a special infrared filter. A number of different types of telescopes are used for celestial imagining.
A telescope that has been optimized as an astrograph will have features such as a fast focal ratio, a wide field of view, and fine focus capabilities. It will also have higher-quality mirrors and include a mount for a camera. Additionally, the secondary mirrors in an astronomical camera are larger than in an similar scope intended for visual observations. Owing to these specialized configurations, an astrograph will not perform well other than for imaging.
As they are special purpose instruments, they often do not support the use of eyepieces for visual observation. An astrograph is usually equipped with special stabilizers to protect the mirrors and other delicate parts. Often there will be an automated, programmable, or robotic mount that will assist in tracking the objects being imaged.