Telescope domes
 
The conventional method of housing an optical telescope is to provide it with a cylindrical or polygonal building which is covered by a hemispherical dome. Access to the light coming from particular directions in the sky is achieved by providing an aperture, usually in the form of the slit, which can be opened whenever the weather conditions are suitable. All aspects of the sky can be covered by allowing the dome to be rotated.
The design of any dome requires that the structure should not contribute to the disturbances which are apparent in all ground-based telescope images. Efforts are made to prevent any flow of air across the telescope aperture. Extremes of air temperature are to be avoided during the daily cycle and the design should ensure that the building does not act like a greenhouse.
As an example of design, the dome of the 200-in (5·08 m) Hale telescope at Mt Palomar, USA, approximates to a hemisphere of 41·5 m diameter. It consists of two ‘skins’ with a gap which is filled with crumpled aluminium foil. Air circulation through the gap, provided by vents at the top and bottom of the dome, takes away much of the heat which is absorbed from the sunlight during the day. Thus, by insulation and air circulation, the inflow of heat during the day is kept to a minimum; indeed, the rise in temperature of a telescope mirror during the day rarely exceeds the temperature of the preceding night by more than 2 ^◦C.
With some elegant designs, as in the case of the 200-in, a model of the telescope is provided to act as an analogue computer. Its orientation is made to match that of the large telescope at all times. By a system of sensing devices and servos, it is made to drive the dome, for example, so that the telescope aperture is never obscured by the limits of the dome’s slit. A safety mechanism is also provided so that the main telescope is never allowed to point below an altitude of 5^◦.