1.4 The real Single-Dish Antenna

A telescope, however, is never perfect since mechanical, thermal, and wind-induced deformations of the structure occur, and the optics may be misaligned and/or have production imperfections, for one or the other reason. The resulting effect on the beam pattern is negligible if the corresponding wavefront deformations introduced by these imperfections are small compared to the wavelength of observation, generally smaller than $\sim \lambda/15$; the effect is noticeable and disturbing when the wavefront deformations are large compared to the wavelength ( $\sim 1/4 \lambda$ and larger). The wavefront deformations due to such imperfections may be of systematic nature, or of random nature, or both.