Posted By Paul McCarl
My problem is not in calculating the distance to the focal plane for eyepieces, I have that down, I think. My actual measurements from the current sonotube setup are matching my calculated numbers, so I think I am ok there. The problem is I also want to be able to take pictures with my SLR Canon EOS camera. And I learned a long time ago that there is a big difference between a focuser for an eyepiece and one that works with my camera, even with the T-ring flush against the draw tube. My focuser on the sonotube doesn't let me reach focus on distant objects, although I can take some nice shots of the birds in my neighbor's trees. 8^) So I can't really measure accurately the difference in the focal planes. Now I have measured the difference using my short tube 80 refractor, and it was just over an inch different. So my question is will the relative distance between focus for a particular eyepiece and my camera be the same on my 17.5" 2000mm fl dob as it is on my 400mm fl ST80? if not, how do I calculate it? Any help will be appreciated.