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A design of telescope for observations of the Sun. The example at the Sacramento Peak Observatory is typical. In that instrument, sunlight enters the tower 41 metres (135 feet) above the ground. A further 67 metres (220 feet) of the telescope lie below ground. The entire optical path is virtually air-free, to avoid distortion of the solar image that would be caused by the presence of hot air. In the observing room an image of the Sun 51 centimetres (20 inches) in diameter is produced with a resolution better than a quarter of an arc second. Sunlight can be directed into spectrographs or other instruments by tilting the main mirror at the bottom of the central tube. The entire optical system is suspended near the top of the tower on mercury float bearings.
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