refractor alignment

[lhuchison]

Heres a question for any technical guys out there, if a focus tube on a refractor looks out of alignment i.e. the laser dot is not central, about 2" away from central on a 4" scope, when placed in the tube without the diagonal. what effect would this have on viewing?

[John]

Assuming your laser is itself aligned (worth checking because many are not !) the laser dot should exit through the centre of the objective lens or as close to it as possible. That means that the focuser is aligned with the optical axis of the scope which the first stage of refractor collimation. Once you have that you can check the tilt of the objective lens using a cheshire collimation eyepiece.

To check your laser collimator is aligned this method is useful:

http://www.cloudynig...php?item_id=520

To check the collimation of the refractor objective lens I use this approach with a cheshire collimator:

http://spacealberta....r/collimate.htm

A 2" error in the centrering of the focuser would be a large one in my experience. In F/10 scopes a few mm here or there seems to have little impact though.

[Merlin66]

I agree!

You should be able to re-align the focuser to bring the laser spot back to the centre of the objective.

[lhuchison]

how do re-align a crayford focuser? i can,t see any adjustment screws?

[John]

You will need to find a way of loosening the focuser where it attaches to the tube and shimming before re-tightening it to align it with the optical axis of the scope. Are we talking about your TAL 100RS here ?.

I'm surprised that it's that far out to be honest. TAL 100's are quite well engineered scopes.

[lhuchison]

Yes it is the TAL John. when i looked down the main tube from the objective lens ,i could see that the focus tube wasnt aligned.

I then tried a laser collimator and its showing the spot about 2 centimetres from centre. So i took the focuser apart and i looks like the focus tube goes out of alignment when pressure is applied from the tightning screws, which have to be used for the focuser to bite.After i re-fitted the focuser, its better . now only 1 centimetre out. I cant see any way over it apart from a replacement focuser.

[John]

Unless it has provision for collimation, a replacement focuser might not help much. The Moonlight focusers have this facility but cost as much as the scope !

I'd play around with shimming / adjusting the current focuser to get it as close as possible to central before jumping into a replacement.

[david9091]

nice post

[lhuchison]

2 hours of fiddling has more or less sorted the problem. happy now!

Thanks for all the help.