Astrograph

Astrographs were often used to make surveys of the night sky, and one of the famous projects was Carte du Ciel.

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.

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).

Each telescope may have individually designed non-achromatic objectives to focus the desired wavelength of light which is paired with the respective color-sensitive (black-and-white) photographic plate.

By taking two exposures of the same section of the sky days or weeks apart, it is possible to find objects such as asteroids, meteors, comets, variable stars, novae, and even unknown planets.

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 ecliptic.

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.