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Focuser collimation? Or just tension?

collimation equipment refractor
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#1 AlphaGJohn


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Posted 13 April 2019 - 07:53 PM

Hi Folks,


It's been an interesting few days. I have had some trouble collimating my bigger refractor and finally broke down and bought one of the startlingly expensive Takahashi collimating telescopes (figured I could use it on my RC too--and I offered to rent it out to local club members ;-).


After fussing with the collimation of the objective cell, I had a friend over recently who wanted to see the scope and I stuck my Howie Glatter laser collimation thingy (RIP, Howie) into the focuser and noted that the laser dot on the objective was not centered. I put the concentric circular pattern holographic gizmo on the laser unit and the results were not exactly what you'd hope for--at least 2 of the outer circles projected onto the wall were broken as it was apparently not very concentric with the objective. Hmmm--and drat! (This is all after working on the collimation of the objective cell.)


Well, I fussed around with the focuser a bit and realized that the brass tension screws were quite loose and not even uniformly so. See brass screw in the photo labeled "Tension / Collimation?"; there's 3 of them spaced at 120 degrees around the focuser's flange that it rotates inside of. (I believe this type of focuser is reasonably common. A friend showed me a new Meade he'd just gotten and it was the same type with the collet-style receiver at the eye end of the focuser--he hadn't been able to figure it out: you rotate the part with the little "spokes" to tighten it around the diagonal or eyepiece).




So, finding things a bit loosey-goosey, I stuck a piece of paper over the objective and put the laser unit back in (without the concentric circle attachment) and marked the paper where the spot appeared. Not only not very centered, but as I rotated the focuser through 90 degrees and checked, the spot had moved considerably--moving was expected; I didn't expect the spacing to be quite so wide. Also, if I tightened the rotation lock, it pushed the laser spot further from the center. So, some tightening of the rotation tension screws and I got the laser spot showing a lot closer to the center, but hardly perfect.


Finally, here's the question: are these tension screws also supposed to allow adjusting the focuser to center it along the light path of the scope? Or are they just intended to provide tension on the rotating body of the focuser? They definitely control the amount of tension on the rotatable part of the focuser; and the ends of the screws are nylon or something similar so clearly they're intended to both provide some friction, but still allow the focuser body to turn inside the flange when they're in contact with the focuser body. If they're intended to have a collimation function, then I'll try fiddling some more.


Here's the best I was able to get in the time available with centering the focuser last evening. Marking the laser spot then rotating it through 90 degrees and marking again (all the way around) resulted in about a 9mm on-a-side square.




Of course, with the focuser better centered, the collimation wasn't good as viewed through the Takahashi collimation scope. (Fortunately, my wonderfully handy-with-tools wife helped with the recollimation and that spend up that process a lot.) I'm sure that if I tested the collimation with the focuser rotated around at different angles, the collimation would appear off in all but the position we left it at while redoing the collimation, but that's a frustration to face another day. The concentric circle holographic attachment projects a better-looking pattern, but as you can see, the bottom of the outer circle is not quite as clear as it might be. But, it's a darn sight better than it was.




I'd be very curious to know if you think I should try to improve the collimation of the focuser or figure it's good enough; I'm not sure how much better it could get without replacing it with something with some more adjustment options. The scope is f/8 so it's perhaps more critical than it would be on a scope with a slower f-ratio, yet less critical than with a faster one. I have had good views through it even before I got the Tak collimation scope, so I don't know how obsessed to be about trying to improve it.


Thanks for any suggestions comments you care to add!


(There is a vague possibility we might have at least some sucker holes this evening after dark. It's been a rainy and cloudy spring so far here.)






Edited by AlphaGJohn, 13 April 2019 - 11:41 PM.

#2 AntMan1


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Posted 13 April 2019 - 07:57 PM

The screw in question is a rotation tension screw. The more weight you add to the imaging train the more you will need to tighten them or you will get tilt..  There should be two more around the collar no?

And they should be turned in the same amount each or that could introduce tilt also.

Edited by AntMan1, 13 April 2019 - 08:00 PM.

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