It would be ideal if stars became true points at perfect focus, but that s not the case. The image of a star at the eyepiece end of a telescope is actually a small disc. See the sidebar Diffraction Effects for details on diffraction. Turbulence in the atmosphere causes scattering of light. The amount of scattering varies from night to night, even from hour to hour and minute to minute.
Every telescope has at least some level of optical aberration.
This interferes with achieving perfect focus. Many telescopes have adjustable collimation. If collimation isn t perfect, you won t be able to achieve perfect focus no matter how hard you try. Chapter 3 provides detailed help for collimating the telescopes most commonly used for CCD imaging. On any given night, one or all of these factors will dominate.