Nasmyth telescope

A comparatively small tertiary flat mirror reflects the light to one of the sides of the telescope.

(The central hole in the primary mirror may still host a Cassegrain focus if the tertiary can be moved out of the way.)

This has significant advantages for spectrographs and other heavy instruments typically used at research observatories.

The 2.4-meter Automated Planet Finder telescope at Lick Observatory in California supports the use of two Nasmyth foci.

The Sierra Nevada Observatory (OSN) in Spain has two Nasmyth telescopes, including a 1.5-meter diameter aperture one.

Simple Nasmyth telescope
56cm Nasmyth-Cassegrain Telescope at the Walter-Hohmann-Observatory in Essen, Germany