Hi Chuck, Ok. I was thinking that if I add a filter wheel between the back of the scope and the imager the light must travel further out the back of the scope to reach the detector chip. But after reading your posts I realized that in order to get the focal point further out the back one has to move the primary mirror forward. Hence the the focal length is the same both with and without the filter wheel. This, despite the fact that the secondary mirror increases the primary mirror's focal length 5 times. So in the case of a C-14 which operates native at f/11 and 3,910mm focal length if I add a .5 focal reducer between the back of the scope and the filter wheel I end up with f/5.5 and 1,955mm focal length. Have I got that right? Thanks for your help, patrick On 08 May 2010, at 01:47, Chuck Hards wrote:
Now, if you use eyepiece projection, you are changing the image scale by increasing the projection distance, and must therefore calculate Effective Focal Length (EFL), but the system focal length remains unchanged.