Actually, there is an optimal spacing for the primary-secondary distance- and the further the spacing departs from this, the more abberations are introduced. But for small variations, the abberations are not usually noticed at f/10 or f/11 (typical SCT ratios). On 5/16/07, daniel turner <outwest112@yahoo.com> wrote:

When I say "generous" that may be an understatement. You see with a refractor or a newtonian the focal length is fixed and the backfocus is constrained to a certain distance from the primary optics. With the SC the focal length is VARIABLE and depends stongly on the distance between the primary and secondary. This is how focus is achieved. It also means that focus can be achieved at any distance behind the primary all the way to infinity, which is why the SC is ideal for the astroimaging. The drawback is the slow focal ratio which grows with the backfocus. That's why I sugested a focal reducer.