An active surface is a surface of a radio telescope that is under active computer control of its shape. Large (more than 10 m in diameter or length) radio telescopes always bend during operation, due to their enormous weight and the fact that even the strongest materials are not perfectly stiff. This bending, in the range of a few millimetres, does not affect low frequency operation much, but dramatically reduces the efficiency of the telescope at higher frequencies where the wavelengths are comparable to the distortion. Typically, the efficiency of a telescope drops appreciably when the deviation from the desired shape is more than 1/10 of the considered wavelength. An active surface uses numerous small actuators to move the surface panels with respect to the underlying frame, and thus maintain the correct shape. An active surface can try to compensate for many different types of errors. The first is gravity—this is simplest since previous measurements, or even a mathematical model, can be used to predict (and correct) any bending. More difficult is correction for wind and thermal errors, since these require measuring and correcting in real time. Some examples of active surfaces are: *

Green Bank Telescope The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Radi ...

Large Millimeter Telescope The Large Millimeter Telescope (LMT) ( es, Gran Telescopio Milimétrico, or GTM) -officially Large Millimeter Telescope Alfonso Serrano ( es, Gran Telescopio Milimétrico Alfonso Serrano)- is the world's largest single-aperture telescope in its f ...

Sardinia Radio Telescope The Sardinia Radio Telescope (SRT) is 64-metre fully steerable radio telescope near San Basilio, Province of Cagliari, Sardinia, Italy. Completed in 2011, it is a collaboration between the Istituto di Radioastronomia di Bologna, the Cagliari Obs ...

* Noto Radio Observatory The Chinese

Five hundred meter Aperture Spherical Telescope The Five-hundred-meter Aperture Spherical radio Telescope (FAST; ), nicknamed Tianyan (, lit. "Sky's/Heaven's Eye"), is a radio telescope located in the Dawodang depression (), a natural basin in Pingtang County, Guizhou, southwest China. FAST ...

uses a uniquely ambitious form of active surface, not only correcting errors, but applying deflections of up to in order to aim and focus the telescope.

References

* *. Discusses many practical details of an active surface. *{{cite news , title=World's largest radio telescope will search for dark matter, listen for aliens , first=Dennis , last=Normile , date=26 September 2016 , journal=Science News , url=http://www.sciencepubs.com/news/2016/09/world-s-largest-radio-telescope-will-search-dark-matter-listen-aliens , doi=10.1126/science.aah7346 Radio astronomy Astronomical imaging Astronomical instruments