Most optical observatories are sited remote from human habitation, usually at the tops of mountains. This siting results from the need for dark, clear skies. At high altitudes the chances of cloudless skies are improved because the observer’s view is usually above the cloud layer.
Choosing a permanent site for an observatory can be difficult. The Royal Greenwich Observatory, for example, was originally located at Greenwich, in London, but as London pollution worsened and the number of street lights gradually increased, conditions for observing the night sky became intolerable. Between 1948 and* 1957, the observatory was moved south and re-established in the grounds of Herstmonceux Castle, near Hailsham in Sussex. Unfortunately, it was not long until atmospheric pollution began to put pressure on this establishment too. An extensive search led to the choice of a volcanic peak on the island of La Palma, in the Canaries, so that the observatory could be based high above the clouds at 7,765 feet (2,367 meters) above sea level.Telescopes
Most of the telescopes in observatories use reflecting optics and are of the Cassegrainian design, in which a secondary convex mirror
reflects the incoming light back
through a hole in the center of the main mirror. Large refractors are
less common, because of the problems of lens sag and the long length of
tube required.
Many modern telescopes are fully automatic, such as
the Anglo-Australian Telescope (AAT), which has a pointing accuracy of
one second of arc. Most telescopes are on equatorial mounts (one axis of
the mounting lies parallel with the earth’s axis of rotation). To track
an object as it moves across the sky, it is necessary only to rotate
the telescope slowly round the mounting axis. This method of tracking,
however, leads to large, heavy, and expensive mountings. Modern mounting
designs simply have a vertical and a horizontal axis (the alt-azimuth
mount), both of which are driven by motors.
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