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The Cassegrain Nasmyth telescope has the same optical design as the classical Cassegrain telescope except for a small tertiary mirror which sends out the light on the side of the telescope tube instead of through a hole in the primary mirror. This design is common in large telescopes, e.g., the W. M. Keck Observatory.
Figure 1 shows the light-path from a star based on the 0.61m telescope in Oslo. Figures 2 and 3 show the use of a focal reducer, flip mirror unit, ocular and CCD camera. See also: Nasmyth telescope.