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EdgeHD8 Collimation - Seccondary Mirror back to sqare One

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#1 vaticanaddict

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Posted 26 September 2020 - 03:31 PM

Hi all.
For sometime now my collimation is... well...not 100%. Durring my last collimation I played with only one Bob's Nob (when I swapped the original screws I did it perfectly by the book - one by one). When testing (after last collimation) I found out the scope was not collimated. When taking the OTA off the mount, I could hear the Seccondary was a bit loosse. After touching slightly the Bob's Knobs I noticed they weren't tighten and that this is what caused the loose in the seccondary.
I hand tightened the Bob's Nobs just till I could feel the tention and stopped. No the seccondary doesn't make any loosse sounds.
My question is - how do I start collimating it from scratc?
I don't know if after hand tightening the knobs - they are eavenly pushing against the seccondarry.
Now all three knobs are hand tight.
Do I start collimating from this point?
Is the Seccondarry not "evenly" mounted?
What should I do to collimate from here (what is point "zero" from where I should start collimating?
Thanks
Eyal J.Y.

#2 Louisv28

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Posted 27 September 2020 - 09:57 AM

  All the screws must be snug, finger tight is the word I would use...not hand tight.

 
  Never start by tightening. All screws are already tight when you start.

 

  Every time you loosen a screw (very little, 1/8 turn), you tighten the 2 others so that all screws are always snug.

 

  Likewise, If you want to tighten one screw, first loosen the two others. Do not succumb to the temptation of over tightening a screw because “its almost there”...
 

Good refs for collimating:

 

https://asterism.org...ain-telescopes/

http://www.astrophoto.fr/collim.html

https://youtu.be/uJ1u3eHBYVY



#3 vaticanaddict

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Posted 28 September 2020 - 12:06 PM

Louis, thanks so much.

#4 vaticanaddict

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Posted 28 September 2020 - 12:06 PM

Louis, thanks so much.

#5 gnowellsct

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Posted 28 September 2020 - 03:36 PM

This is specifically for when the scope is grossly out of collimation

 

It is not inconceivable that you have the wrong screws.  There are many versions of Bob's knobs as the designs over the years have varied a bit.

 

I would go back to your regular screws if you have them and collimate with a tool and then leave it alone except for the occasional touch up.  Bob's knobs can be OK, but they can also be more trouble than they are worth.

 

Greg N



#6 Astrojedi

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Posted 29 September 2020 - 03:48 PM

I would go back to regular screws. Once you collimate and tighten the collimation screws the SCT should not need collimation for months / years. The last time I collimated my EdgeHD 8 and 9.25 scopes was 1.5 years ago. They are still holding critical collimation even after trips to dark sites on bumpy roads.


Edited by Astrojedi, 29 September 2020 - 03:50 PM.

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