Quote:
Originally Posted by Kunama
John, If your extension tube is actually 115mm (As long as you didn't include the inserted nosepiece in you measurement  ) then you scope has a back focus of 201mm which agreed with the 12mm difference between the Nagler T6 and Delos 17.3 (actual difference should be 12.2mm or 0.48")
I was under the impression that CFF measured the back focus from the back of the fitted adapter on the draw tube. |
Thanks Matt for confirming these eyepiece figures. My extensions are a 35mm + 80mm and I am not including the nosepiece.
The backfocus should be measured as you say, from the fitted adaptor (3.5" to 2") in the draw tube.
I have just built a projection rod to bolt to the scope and I will project the sun or moon sometime.
Darren, I do not see how measuring the focal length from the objective will help - it is difficult to know the exact point on the objective to measure and where it is. But you are right in thinking I need to know the F.L. - I am starting to believe it is closer to 935mm rather than the stated 915mm.