Zenith telescope

They are used for precision measurement of star positions, to simplify telescope construction, or both.

Extremely sensitive levels are attached and the telescope has an eyepiece fitted with a micrometer.

[1] They are used for the measurement of small differences of zenith distance, and used in the determination of astronomic latitude.

High-precision (and fixed building) zenith telescopes were also used until the early 1980s to track Earth's north pole position e.g. Earth's rotation axis position (polar motion).

The NASA Orbital Debris Observatory with an aperture of 3 m and the Large Zenith Telescope with an aperture of 6 m are constructed as zenith telescopes, as the use of liquid mirrors limits them to pointing straight up.