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Secondary collimation screws to hard to turn

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#1 Mitchell Duke

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Posted 23 November 2012 - 09:27 AM

I just collimated the scope after trying to fix some astigmatism in the C14. Well I did not improve the astigmatism and now the collimation screws are too hard to turn. Anyone have any advice on the collimation screws, and any advice on the astigmatism?

#2 orion61

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Posted 23 November 2012 - 09:30 PM

Is the scope new to you/
What did you do to " FIX" the astigmatism issue?
Do you have pix? has it always had the problem?
You should be able to unscrew them all 1/16 turn or so.
interesting, this is about the 3rd one of these I've heard about this last month or 2 for astigmatism..
Did you change anything? Tube etc, cleaning?

#3 Mitchell Duke

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Posted 23 November 2012 - 10:31 PM

Thanks for the reply, I was wondering if anyone could help with some ideas. The scope was new when I purchased it three years ago. The infocus high power airy disk is broken into four lobes. I took apart the scope to install peltiers, and it seems that was when the problems started. I was very careful to mark everything I took apart so that it would be installed back to the factory setup. The baffle seems to be centered under the secondary, but im still wondering if it might be off centered. I did the out of focus star test moving the disk around the perimeter of the field of view and it showed no cutoffs as Ed said to do. My plan is now to do a test with an artificial star to see if it shows astigmatism to rule out possible seeing as the culprit. If that does not work I will purchase the Hotech advanced CT collimator as I heard it will ensure alignment of the complete optical system. The collimation knobs seem a little harder to turn than I recall in the past but they do tur 1/16. Any ideas or suggestions are welcomed... I will attach the star test again to show the current issue, as well as a Jupiter shot to show how great the optics were.

Thanks,
Mitchell

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#4 Mitchell Duke

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Posted 23 November 2012 - 10:32 PM

Star test after modifications

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#5 DesertRat

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Posted 23 November 2012 - 11:32 PM

Mitch,
It can be problematic to judge whether or not there is astigmatism (AST) by viewing a stacked in focus image of a star. This assumes we are talking about a small amount of AST, a large amount would of course be viewable by eye. As you go thru focus you would see a small line turn around in orientation, and a small cross at focus. In average seeing you might see a small oval turn around about the focal point. Of course you have to wear glasses if you have astigmatism in your eye!

If you could post an image of a bright star inside and outside of focus it would help diagnose the problem. Without a barlow using a red or green filter take a video of a bright star out of focus (inside and outside the same amount) so that it covers 100-128 pixels or so. Its preferable to use an external focuser for this. Then we can run it through Roddier and see what it says. You can PM me or email your results for analysis - if you want to.

I've seen plenty of nights that gave some indication of a small amount of astigmatism by stacking star images on a scope with little astigmatism (every scope has a little, but it should be less than 10nm rms and hopefully smaller than that). I've seen stacking programs exhibit this for different sized align boxes.

Best wishes,
Glenn

#6 Tom and Beth

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Posted 23 November 2012 - 11:39 PM

I just collimated the scope after trying to fix some astigmatism in the C14. Well I did not improve the astigmatism and now the collimation screws are too hard to turn. Anyone have any advice on the collimation screws, and any advice on the astigmatism?


Hello Mitchell,

Others can explain the Secondary housing construction better, but I bet you that your astigmatism and the screws being too tight are related.

Think of the back of the Secondary housing as being cone shaped. If you collimated by always TIGHTENING, eventually the housing can warp the secondary mirror out of shape. You need to back off all three screws, start with an 1/8 turn. When you re-collimate, do so by first loosening one side, and a little bit in on the other. A little here, a little there. If any of your screws free turn (no load at all) then just bring it back until it makes contact.

#7 orion61

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Posted 23 November 2012 - 11:59 PM

My 12" Meade had a collimating issue when I got it, Bobs knobs YUK! but there was another problem, seems someone tightened the screws down so much at one time it smashed down the center peak in the secondary.
I simply bought the smallest strong spring I could find,
Superglued it on the back and reassembled it (after letting it dry 3 days) Super glue outgasses something terrible and I'll never use it again..
BUT the final outcome was great! it did clear up a slight deformation. The origonal owner said he would have never sold it had he known it would be that easy to fix.
I used to have to monkey around for 15 min. EVERY time I used it, It has been a year and 2 months now since I touched it.... BTW goodby Bobs knobs... they are ok for some, but not me.

#8 Mitchell Duke

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Posted 24 November 2012 - 08:57 AM

Thanks for the ideas!!

Glen next good seeing night i will send you the shots of the star test.

#9 rmollise

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Posted 24 November 2012 - 09:02 AM

I recommend Bob's Knobs highly; they are just great...IF... They are installed properly, which only takes reading the cotton picking directions. :lol:

#10 orion61

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Posted 24 November 2012 - 10:26 AM

There are 3 small washers that must be installed with them,
I have wondered why they aren't locking type or the Bobs knobs dont have the semi locking compound on them to keep from moving over time.
I just shudder every time I see a dust cover put on with those things JUST under it, A slight bump or laying someting on an ota.. you can keep em.. A properly collimated stock set has done me well for 3 decades..
I have had them, but I havent had the best luck with them, they had to be messed with every time or every other time
I used the scope.
If they could fix the creep and lower the profile another 1mm I would try them again. it is a good idea.
I have thought of a small wireless collimating system would be cool ecpecially for Large SCT's, or wired on Newtonians.
oops there goes my first million....

#11 rmollise

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Posted 24 November 2012 - 04:02 PM

There are 3 small washers that must be installed with them,
I have wondered why they aren't locking type or the Bobs knobs dont have the semi locking compound on them to keep from moving over time.
I just shudder every time I see a dust cover put on with those things JUST under it, A slight bump or laying someting on an ota.. you can keep em.. A properly collimated stock set has done me well for 3 decades..
I have had them, but I havent had the best luck with them, they had to be messed with every time or every other time
I used the scope.
If they could fix the creep and lower the profile another 1mm I would try them again. it is a good idea.
I have thought of a small wireless collimating system would be cool ecpecially for Large SCT's, or wired on Newtonians.
oops there goes my first million....


Lock washers of any sort are not needed. If the proper screws are properly installed and collimation is done correctly, there is no "creep."

#12 Mitchell Duke

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Posted 25 November 2012 - 09:12 AM

Im going to try an artificial star to test the optics tonight. I should be able to get the artificial star around 200 feet from the scope. This way i can eliminate bad seeing and possibly the thermals better.






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