Astrograph

Astrographs (also astrographs ) are special cameras that are used for scientific astrophotography. You have the ability, in spite of long focal lengths to capture large fields in the sky and bright light map.

For this task their lenses are a particularly large supply (eg 300 mm × 300 mm), distortion-free as possible and plan field. Also, the imaging scale over a wide spectral range should remain constant and do not change too much with nocturnal cooling. With lens lenses optics were already at the penultimate turn of the century constructed with 40 cm opening ( see picture of Bruce double astrographs ).

Double astrographs

Especially double astrographs were a useful tool of practical astronomy. They were produced until the second half of the 20th century. Two identical cameras are accurate by a equatorial mount the stars tracked. Different features of the two astrographs with color filters or with a prism in front of one of the two lenses can be made with different information at the same time comparison shots of the same area of ​​sky, but. This gives photos of the same astronomical observation objects that differ only in color or a normal photo and one on which the stars were pulled apart to spectra. It is also contemplated, one of the photographic plates to move relative to the entire instrument. Using this method one obtains again a normal photo of a particular sky area on which a comet or asteroid represented as a bar and a second track on which these subjects will be sharp and the stars as dashes. Since the beginning of astrophotography the second plates also serve to confirm to misinterpret not drive failure or reflections as newly discovered celestial bodies.

Optical construction

At the beginning of the 20th century, lenses were used for astrographs that resembled the former photographic lenses under construction. These were then 3 -element lenses symmetrically constructed. Between two biconvex lenses there was a biconcave lens. For the photograph of this system was further developed by changing the rear lens in a cemented lens composed of two different glasses to the known Tessar. The optical designer August Sonnenfeld went in 1932 for large astrographs lenses another way. He split the front lens into two identical biconvex and was thus an almost ideal optical system, the Astro- Vierlinser.

Today in astrophotography for the same tasks Schmidt cameras and Ritchey -Chrétien Cassegrain telescopes are used, which also have a very large field. Refracting telescopes are now less used in scientific astronomy. The principle of the binoculars is obsolete, and is used for completely different purposes ( interferometry ).