Image as seen in a D30 viewfinder. Configuration: 24000mm, f/120, eyepiece projection using a 7mm Nagler. Focusing a telescope on a planet is always a very major issue when using a DSLR. Everything is seen mushy at high magnification, at the diffraction limit. While in theory f30 to f50 should offer ample magnification for most DSLRs (Nyquist sampling and all that), focusing is considerably easier with excessive magnification. The above is a full frame. At this magnification you can actually try to focus on the planet's details, in this case the Cassini Division in the rings.