Broken telescope

As a broken telescope are refractors and measurement telescopes whose optical path is deflected by 90 ° to the side indicated. For this purpose, a deflecting prism or an auxiliary mirror is attached approximately half the focal length of the lens, which deflects the light rays through an opening in the tube or a cube on the tilt axis into the horizontal.

Passage instrument (around 1900) with a broken telescope. The deflecting prism is located in the metal cube at the intersection of the three axes

Broken telescopes were already common in earlier triangulation theodolites (e.g. von Starke & Kammerer) and have been so for most passage instruments and large astro-geodetic universal instruments since the 19th century . More recent examples are the Askania AP-70, the Wild T4 and the Kern DKM3-A .

In astronomy, broken telescopes have been used in some equatorial and comet seekers since 1870 . Compare the Coudé beam path .

The advantages of this construction are several:

1. The telescope can be tilted up to the zenith (and beyond), which is essential for stargazing.
2. The side view of the eyepiece is pleasant for the observer.
3. It is also always at the same level.
4. The overall length is shortened, which also reduces the effects of telescope bending and changes in air temperature .
5. Because of the beam path running in the hollow tilt axis, it remains accessible for attaching rider or hanging vials .