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Collimation after swapping focuser?

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#1 DaveG

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Posted 04 April 2013 - 06:44 PM

So I purchased a GSO two speed focuser to replace the single speed focuser on my new Vixen ED80sf. It seems to be a fairly simple process of swapping focusers, but has anyone had to collimate their scope after doing so?

#2 GOLGO13

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Posted 04 April 2013 - 07:20 PM

I think unless there was something wrong with the collimation before you swapped the focusers, no collimation should be necessary.

#3 DaveG

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Posted 04 April 2013 - 07:44 PM

Sounds good to me. Collimation is spot on right now.

#4 kevint1

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Posted 06 April 2013 - 07:47 AM

I've replaced the stock focuser on two refractors with Moonlites, without any resulting collimation issues. On the first scope, I think the collimation actually improved.

#5 Scott in NC

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Posted 06 April 2013 - 09:17 AM

I think unless there was something wrong with the collimation before you swapped the focusers, no collimation should be necessary.


+1

If there was a noticeable degradation in the in-focus or out-of-focus star images after swapping focusers, my first inclination would be to make sure the focuser was lined up correctly on-axis and didn't need shimming. I wouldn't mess with the lens alignment to try to "correct" an issue that occurred after changing the focuser.

#6 DaveG

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Posted 06 April 2013 - 05:26 PM

Well, after swapping the focuser the collimation is absolutely perfect, maybe even better than what it was before. I placed the original single speed crayford on my Celestron 102GT and the collimation is perfect on it as well. Really happy and the GSO dual speed looks fantastic on the ED80.

#7 Scott in NC

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Posted 06 April 2013 - 06:24 PM

:waytogo:

#8 jbalsam

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Posted 09 April 2013 - 01:31 PM

This response is meant for anyone who comes here with a similar question and would like a basic understanding of why they might be having this problem (clearly your scope didn't have a problem so an explanation isn't necessary :grin:).

*begin professorial pontification*

The purpose of collimation is to get the primary axis of your lens to be parallel with the primary axis of your eyepiece (or parallel to the surface normal of your CCD chip). Switching the focuser could cause collimation to change if there's enough play in how the focuser attaches to the scope.

This is also why you might notice collimation differences when you attach a camera that you didn't notice when you were observing: if your eyepiece holder and your camera don't attach to your focuser in such a way that their axis are very close to parallel, you'll get noticeable differences in collimation.

A quick napkin calculation: if an 80mm scope uses typical 1/4"-20 push-pull screws for its collimation, and if during collimation you are sensitive to screw rotation of 1/16th of a turn, then you're sensitive to a .05 degree deviation in the parallelism of these two axes of your scope. That's not a very large angle, and if you have a focuser that's not shall we say "precision"... it's not hard to get deviations like that. I know that if I loosen the screws which hold my focuser assembly to the sheetmetal tube of my scope I can wiggle it around and re-tighten it in any number of fantastical and possibly non-Euclidean angles which its designers may not have expressly intended.

*end professorial pontification*






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