Large radio telescopes (more than 10 m in diameter or length) 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.
The first is gravity—this is simplest since previous measurements, or even a mathematical model, can be used to predict (and correct) any bending.
Some examples of active surfaces are: The Chinese Five hundred meter Aperture Spherical Telescope uses a uniquely ambitious form of active surface, not only correcting errors, but applying deflections of up to 47 centimetres (19 in) in order to aim and focus the telescope.