Radio telescopes
 

Extremely large constructions are required for the mountings of radio telescopes and, consequently, not every telescope is given adjustment to allow all points of the sky to be monitored. In some cases, however, it has been found possible to design the telescope so that it can be mounted equatorially. The mechanical problems for this type of mount are more severe than for alt-azimuth mountings and the larger telescopes with dish collectors are more often mounted by this latter method. The following of a celestial radio source is sometimes achieved by using a model which is mounted equatorially, providing an analogue computer. The model provides driving currents to the motors attached to the axes of the alt-azimuth mounted telescope and the system is servo-controlled so that the model telescope and the real telescope are simultaneously pointing to the same direction on the celestial sphere. An example using this system is the 250 ft (76 m) Lovell radio telescope at Jodrell Bank, England. In passing, it may be mentioned that the mechanical problems of mounting the 6 m optical telescope of the Russian Academy of Sciences in the Caucasus Mountains, have been solved in the same way. This telescope is mounted on alt-azimuth axes and is controlled by an analogue computer.
A few radio telescope dishes have been designed so that they remain in a fixed position and the skies are scanned by allowing the diurnal motion to carry a strip of the celestial sphere through the focus of the telescope. By tilting the antenna supports, so that the sensor is not at the exact focus of the dish, a range of strips can be scanned. In general, however, the area of sky that is available for observation is greatly reduced by using this type of fixed system. Nevertheless, the fixed-dish design allows extremely large collecting areas to be used. By choosing the observing station carefully, it is possible to allow the natural contour of the land to be used in providing the basic shape and support of the telescope dish. This idea has been used, for example, for the 1000 ft (305 m) steel mesh dish at Arecibo, Puerto Rico. By having an extendible boom to carry the antennas, this particular telescope allows a range of approximately 45◦ in observable declinations.
Observations in radio astronomy are not as weather-dependent as they are for the optical region and telescopes are protected from the weather only by paint and grease. It is usually unnecessary to provide them with a retractable cover in the same way that an optical telescope is protected by a dome. However, because of their large size and exposure, it is important that they are designed to be stable in high winds. According to their design, a limit is usually set to the wind speed for which the telescope can be operated.

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