Zenith prism

The diagonal prism or the diagonal mirror is an additional device for astronomical and geodetic telescopes for observing stars close to the zenith and steep terrestrial sightings .

With the astronomical lens telescope and the Cassegrain reflector telescope , the beam path is deflected to the side by 90 ° by a precisely ground, right-angled glass prism and the eyepiece is attached behind it. The simpler construction is the zenith mirror, which instead of the prism has a small optical plane mirror which, however, just like the prism, has to be adjusted.

Diagonal mirrors are usually available in 1.25 and 2 inch jacks. Modern, dielectrically coated zenith mirrors have a high optical quality; color errors such as those that can occur when using prisms in the beam path are avoided.

The convenience of looking down slightly instead of steeply upwards is paid for with a reversed image, which is annoying when observing the moon , for example . Without a prism, the image of the astronomical telescope is wrong, but turning the moon or star map is not a disadvantage if it is transparent.

Presumably the star diagonal was constructed by the English astronomer George Biddell Airy . In 1847 he equipped an equatorially mounted telescope with it.

In the middle of the 20th century, instead of the simple prism, special zenithoculars were developed for theodolites , which compensate for the lengthening of the beam path with a system of collecting and diverging lenses. To use it, unscrew the telescope eyepiece, attach this insert and screw the eyepiece (which also contains the reticle plate) back on.