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Do I have a Collimation Issue?

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#1 langman78

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Posted 07 May 2014 - 08:38 PM

Hi,
I was out last night doing some visual with my 8se and something caught my attention while I was in the process of doing my 2 star alignment. My first star for alignment was Sirus and bringing my OTA into focus I noticed my scope could be out of collimation, but I’m not too sure.
The reason for me not being too sure is (out of focus) the central donut ring looks to be quite central and good but the outer ring seems more like a 80 to 95% full moon type of effect or a section of the outer ring is missing. Is this a collimation issue? If so how do I correct this? I am familiar with the collimation process for my old newt scope as I did it every night before going out, but not so sure with a SCT. I have heard Bob’s Knobs mentioned on here plenty of times on here as a collimation aid but I have noticed 3 or 4 screws on the secondary mirror plate and wondering if it’s a matter of tweaking these screws to get good collimation?
Thanks for reading and I hope to get a reply soon.
Regards,
Ben

#2 hopskipson

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Posted 07 May 2014 - 08:57 PM

First I would check to see if you had your dew shield on properly. If it is off kilter it will clip the outer diffraction ring. Now if the ring was whole but lopsided then you need to collimate with those three little screws or replace the screws with knobs and do it by hand. I don't like any metal implements near my corrector plate, thank you very much!!!

#3 redlinedb16a

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Posted 09 May 2014 - 07:42 AM

also collimate without the use of a diagnol

this vid is helpful
https://www.youtube....h?v=hqRVIDj4aZA

#4 Digital Don

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Posted 09 May 2014 - 06:26 PM

also collimate without the use of a diagnol


Unless you normally do not use a diagonal while observing, the diagonal should be in place while collimating. Once you begin the collimation process, be sure you do not rotate the diagonal. Since the diagonal itself is probably not "perfect" doing so will alter the center of the field and change collimation.

Don:usa:

#5 redlinedb16a

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Posted 09 May 2014 - 06:42 PM

Ive hear both with and without,I collimate my diagnols so either in or out is ok for me.

if your diagnol isn't properly aligned then that could pose issues as well.

#6 mostlyemptyspace

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Posted 12 May 2014 - 08:54 AM

What I would do is remove any unnecessary glass and accessories that may affect what you see. So remove the diagonal, dew shield, focal reducer, flip mirror or whatever else you have in your optical train.

Then, try different eyepieces to see if you get the same result.

Try another star, just in case something is blocking your view.

Then I would make a very slight adjustment to one collimation screw, and see if it has any effect on what you're seeing. If not, reverse the adjustment.

#7 Amaranthus

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Posted 16 May 2014 - 11:38 AM

Have you tried testing your collimation with a Duncan Mask? Put in a high power eyepiece with the mask, and it becomes immediately clear if you have a problem. They are dead simple to make with some stiff cardboard.






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