Nasmyth telescope
A Cassegrain variant with side-exit light for heavy instruments.
The Nasmyth telescope, also known as the Nasmyth–Cassegrain or Cassegrain–Nasmyth, is a type of reflecting telescope invented by Scottish engineer James Nasmyth in 1845. It is a variation of the Cassegrain reflector, where the light path is redirected sideways to an eyepiece.
In its design, light first hits a concave primary mirror and is then directed toward a convex secondary mirror. A small, flat tertiary mirror then reflects the light to one side of the telescope. (If the tertiary mirror can be moved aside, the central hole in the primary mirror can still provide a Cassegrain focus.) This flat mirror sits on the altitude axis, so the beam exits through a hole in the center of the altitude bearing. As a result, the eyepiece or instrument does not need to move up and down with the telescope; the tertiary mirror's angle adjusts based on the telescope's pointing and the star's elevation. This allows heavy instruments, like spectrographs, to be used without unbalancing the telescope or adding strain to the altitude bearings—a major advantage for research observatories. Most modern research telescopes can be set up to use the Nasmyth focus.
Notable observatories using this design include the twin 10-meter telescopes at the W. M. Keck Observatory in Hawaii, the 8.2-meter Subaru Telescope nearby, the four 8.2-meter Unit Telescopes of ESO's Very Large Telescope, and their predecessor, the 3.6-meter New Technology Telescope. The future Thirty Meter Telescope and Extremely Large Telescope also plan similar setups. The 2.4-meter Automated Planet Finder at Lick Observatory in California supports two Nasmyth foci. Spain's Sierra Nevada Observatory (OSN) hosts two Nasmyth telescopes, including one with a 1.5-meter aperture. The SOFIA airborne telescope also uses this design.
- Inventor
- James Nasmyth
- Year developed
- 1845
- Also known as
- Nasmyth–Cassegrain or Cassegrain–Nasmyth
- Type
- reflecting telescope
- Notable examples
- Keck Observatory twin 10-meter telescopes, Subaru Telescope (8.2-meter), VLT four 8.2-meter Unit Telescopes, New Technology Telescope (3.6-meter), Automated Planet Finder (2.4-meter), Sierra Nevada Ob
Lore & Background
The Nasmyth telescope was developed by Scottish inventor James Nasmyth in 1845 as a modified form of the Cassegrain reflector. In its scheme, light falls on a concave primary mirror, then is reflected towards a convex secondary mirror. 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 flat mirror is placed on the altitude axis, so that the beam exits through 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, as the tertiary mirror's angle with the main telescope axis is adjustable as a function of the telescope's pointing and the star's elevation above the horizon. Therefore, a heavy instrument can be used without upsetting the balance of the telescope or increasing the load on the altitude bearings.
Reader's Guide
The Nasmyth design 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.' Notable examples include the twin 10-meter telescopes at W. M. Keck Observatory in Hawaii, the 8.2-meter Subaru Telescope, the four 8.2-meter Unit Telescopes of ESO's Very Large Telescope, and their predecessor the 3.6-meter New Technology Telescope. These support an array of specialized instruments on their Nasmyth platforms, with a similar design being used for the future Thirty Meter Telescope and Extremely Large Telescope. The 2.4-meter Automated Planet Finder telescope at Lick Observatory supports two Nasmyth foci, and the Sierra Nevada Observatory in Spain has two Nasmyth telescopes, including a 1.5-meter diameter aperture one. The SOFIA airborne telescope also uses this design. The legacy of the Nasmyth telescope is its enabling of heavy, stable instrumentation at major observatories worldwide.
Did You Know?
- It uses a tertiary flat mirror placed on the altitude axis to reflect light sideways.
- The design allows heavy instruments to be used without upsetting the telescope's balance.
- The twin 10-meter Keck telescopes and the 8.2-meter Subaru Telescope use Nasmyth foci.
More in Telescope Types 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced