How well does the Series IV work with newtonian reflectors?
A. It depends on the telescope’s optical design. Knife-edge focusing is extremely effective for focusing SCTs with longer focal ratios, but falls short in some respects for small newtonian reflectors with low f / ratios. The problem usually boils down to limitations in the telescope design itself. All optical system designs are optimized for use over a fairly narrow range of focusing. This distance is roughly the standard length of a t-adapter (55mm) , plus an allowance for a typical camera’s film/plane distance (30mm-80mm). In the case of an SCT, this design range usually extends about 20 centimeters behind the telescope and there are no problems focusing. In a newtonian design, however, this distance is intentionally made much shorter to minimize the effects of the secondary mirror obstruction. The result is that there may simply not be enough room to accommodate the additional length adapters and cameras take up without moving outside of the optimal design range. This may mean that
