ESO VLT: II. Optical specifications and performance of large optics

Up to now telescope optics were usually specified in terms of geometrical errors which cannot be linked to the actual performance under atmospheric turbulence limitation. A more realistic approach is proposed which takes into account atmospheric seeing and diffraction. The main advantage of the method is that at the same time it describes the final performance of the telescope, and gives to the optical manufacturer the maximum freedom to define and possibly modify its own manufacturing error budget.