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
; the effect is noticeable and disturbing when the
wavefront deformations are large compared to the wavelength (
and
larger). The wavefront deformations due to such imperfections may be of systematic
nature, or of random nature, or both.