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reaching focus with focal reducer and external foc

Started by John999R, 01/22/2013 07:44PM
Posted 01/22/2013 07:44PM Opening Post
I have a CPC1100 and recently installed a Celestron f/6.3 reducer for visual use. The optic train also includes a William Optics focuser that I really like because it solved the image shifting associated with the OTA's focuser. However, I cannot now reach focus and I don't know if there is anything I can do about it because I've been told the eyepiece has to be approx 100mm behind the focal reducer to reach focus. That means I need to reduce the distance and I don't see any resolution without removing the WO focuser and replacing it with a shorter extender. I did that and was able to reach focus down to almost an 11mm EP. Still not satisfactory in my book and I could reduce the distance more if I could find an extender/adaptor that is shorter by about 3/8". If I were able to get down the additional 3/8" I hope this would enable the scope to reach focus with a range of magnifications, otherwise I'm simply going to remove the reducer and put the scope back to the way it was with the WO focuser. As much as I like the increased field of view and flatter field, I want to maintain the flexibility of having the dual speed focuser and no image shifting.

I suppose my question is has anyone installed a reducer and was still able to reach focus with an external focuser like the WO I have? Seems like an impossible task since you have to stay within 100-105mm distance from reducer to EP. One other thing I neglected to mention and I didn't try the set up without the diagonal, I think doing that reduces length some more, but at the same time adds viewing comfort problems at higher degrees.
Posted 01/28/2013 03:44AM #1

Where are you positioning the focal reducer? If you placed the focal reducer after the focuser, between the diagonal and the focuser, at least you would have the approximate 100mm of spacing.

Otherwise, the problem you are facing is that the effective focal ratio of a focal reducer depends on the distance between the actual focal plane without the reducer and the reducer. When you increase that distance by adding the Crayford focuser, you are decreasing the effective focal ratio which positions the focal plane even further inward. It is a losing battle.