Is there anything on-line which will let me calculate the spacers necessary to focus images onto the DSLR with an eyepiece in the train (my high school optics was 53 years ago!)? I have a standard SW 8" and have successfully used a 25MM EP (Celestron ordinary) in the standard Bintel nosepiece/T ring with my Canon. However no other EP, longer or shorter FL, will come to focus.
