Nasmyth telescope

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The Nasmyth telescope is a reflecting telescope developed by James Nasmyth. It is a modified form of a Cassegrain telescope, mounted on an alt-azimuth mount.

As in the Cassegrain telescope, the light falls on a concave primary mirror, then goes into a convex secondary mirror. However there is no hole in the primary mirror. Instead a small flat mirror reflects the light to one of the sides of the telescope. This flat mirror is placed on the altitude axis, so that the exit beam comes out of a hole in the middle of the altitude bearing. This means the eyepiece or instrument does not need to move up and down with the telescope; therefore a heavy instrument can be used without upsetting the balance of the telescope or increasing the load on the altitude bearings. This has significant advantages for spectrographs and other heavy instruments typically used at research observatories. Most modern research telescopes can be configured into a Nasmyth telescope (i.e. to use the "Nasmyth Focus").