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Some astronomical instruments, e.g. high-resolution spectrographs, are too large, too heavy and too sensitive to flexure to be mounted below the primary mirror of a Cassegrain or Ritchey-Chretien telescope. It is possible, however, to provide a stable platform for an instrument by redirecting the light off the telescope using one or more additional mirrors, as shown in figure 36.

figure 36: Schematic of a Cassegrain telescope showing the use of a tertiary mirror to direct light to a coudé or Nasmyth focus. The same diagram applies to a Ritchey-Chretien telescope, except that the primary would be hyperbolic rather than parabolic. In the case of the coudé focus, additional flat mirrors after the tertiary are sometimes employed to direct the light to a given location off the telescope.

The coudé focus (from the French word for elbow) is usually found on equatorially-mounted telescopes. A typical coudé design uses a flat mirror (the tertiary) to redirect the light along the declination axis of the telescope and then another flat mirror (the quaternary) to direct the light down the (fixed) polar axis into a room near the base of the telescope in which the instrument is mounted, as shown in figure 37. The instrument hence remains stationary whilst the telescope moves. In addition to the light lost at each reflection, the main drawback of the coudé design is that the field of view rotates as the telescope tracks an object, so derotation optics are usually required to correct for this (which causes yet more light loss).

figure 37: Left: A photograph of the ESO 3.6m Cassegrain reflector in Chile. A schematic of the four mirrors and light path of a typical coudé arrangement are overlaid. Right: A photograph of a typical coudé room. The light from the telescope comes through the pipe at the upper-left and falls on the instrumentation on the optical bench.


The Nasmyth focus (named after the 19th-century Scottish engineer James Nasmyth) is usually found on alt-azimuth mounted telescopes. A tertiary mirror redirects the light horizontally along the altitude axis to the side of the telescope. Hence the Nasmyth focus moves with the telescope azimuth axis, but the beam remains horizontal with respect to the ground. Like in the coudé design, the field of view rotates at the Nasmyth focus whilst tracking an object, but this can be compensated for by using derotation optics or by mounting the instrument on a rotating platform which rotates with the field of view, as shown in figure 38.

figure 38: A photograph of an instrument (ULTRACAM) mounted on one of the Nasmyth focii of the 8.2m Very Large Telescope in Chile.

Many modern research telescopes have two Nasmyth foci, one at each end of the altitude axis. The tertiary mirror can then be tilted to direct light to either Nasmyth focus, or retracted to allow light to pass straight through to the Cassegrain (or Ritchey-Chretien) focus. This configuration allows three different instruments to be permanently attached to the telescope, each accessible at the flick of a switch, giving astronomers significantly more flexibility. Some telescopes have horizontal platforms at the tops of the tines, such as the one the person is standing on in figure 38. It is possible to mount particularly large, heavy or sensitive instruments horizontally on these Nasmyth platforms using an optical bench and derotation optics. These instruments are often enclosed in light-tight, temperature-controlled laboratories, such as shown in figure 39.

figure 39: Left: A photograph of the 4.2m William Herschel Telescope on La Palma showing the two laboratories (GHRIL and GRACE) mounted at the two Nasmyth focii. It should be noted that both labs rotate with the azimuth axis of the telescope. The black turret at the centre of the image houses the tertiary mirror. Right: A photograph of the inside of one of the laboratories. Light from the telescope enters the lab just behind where the person is standing, passing through some derotation optics before entering the complex instrumentation (an adaptive optics imager) on the optical bench.

©Vik Dhillon, 3rd September 2010