Telescope alignment: is the zero-coma condition sufficient?

Modern telescopes have demonstrated the possibility of obtaining very high quality images (as good as 0.3 arc sec) from ground-based telescopes. To maintain this quality during regular use places stringent requirements on many other aspects like alignment (traditionally zero coma) and pointing and tracking, especially if active optics is used. We argue that for these stringent conditions to be met, for correcting coma, tilt and decentering have to be measured and corrected separately, both at the initial set-up of the telescope, and during normal operations. Otherwise: (1) a complex pattern of astigmatism can be introduced during the initial alignment phase; (2) pointing changes are introduced during observations, which though possible to correct, can limit the optical quality.