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Collimation of Primary and Secondary mirrors whilst using binoviewers?

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#1 Sarciness

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Posted 25 November 2021 - 10:38 PM

Sorry if it's been asked before but I couldn't find the answer to my noob question after searching.

 

When collimating (the scope, not the binoviewer itself), do I need to remove the binoviewer, insert a monoviewer and then collimate or should I collimate through one of the bino slots? Are there any special tecniques that need to be employed?

 

I am using a binoviewer with a collapsible Dob with a Cheshire / sight tube combination tool. I also have a basic laser collimator available.

 

 



#2 wrvond

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Posted 25 November 2021 - 11:40 PM

Collimation ensures the proper alignment between the focuser, secondary and primary mirror. No viewer should be inserted into the focuser- that’s where you insert the Cheshire or laser.


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#3 Reid W

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Posted 27 November 2021 - 09:25 PM

Sarciness-

 

Before I got my secondary properly positioned, I found that collimation was spot on in monoview, even with an autocollimator.  But when using the binoviewer, collination would still be off.  So I would adjust primary ... usually leaving secondary alone.

 

I recently started over on mirror placements/locations and found that mono and bino collimation was better .. matched 

 

Denk website notes to collimate with binoviewer.  I would collimate on a star then swing to planetary viewing.


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#4 Sarciness

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Posted 29 November 2021 - 12:15 AM

OK, I now seem to have 2 contradicting pieces of advice. Not sure what to do from here... I guess my options are:

 

1. Fully extend telescope (position 2)

2. Insert 1.25 inch adapter

3. Insert Cheshine

4. Align Secondary with focuser and adjust rotation

5. Align primary with secondary

6. Retract the telescope to binoviewer position (position 1)

7. Remove 1.25 adapter, insert binoviewer and eyepieces

8. Check collimation on a star (but if it appears off, is it really off, or is that due to the binoviewer?)

 

or

 

a) Extend the telescope part-way (poisition 1)

b) Insert binoviewer and Chrshire into one of the holes

c) Align Secondary with focuser and adjust rotation

d) Align primary with secondary

e) Check collimation on a star

I feel like wrvond is directing me towards the numbered list and Reid W is directing me towards the lettered list? Any further guidance?



#5 wrvond

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Posted 29 November 2021 - 09:19 AM

If using the BV requires you to reconfigure the scope, then I would collimate it in the position you are going to use it in. Inserting your laser into one side of the BV will certainly allow you to tell if the entire optical train is collimated, but if it's off (according to the laser) what are you going to adjust? 

Personally, I'd ensure my mirrors were collimated to each other then ensure the BV is properly aligned in the focuser, etc.



#6 betacygni

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Posted 29 November 2021 - 10:08 AM

Would be interesting to check the collimation with respect to both methods, out of curiosity, and see if they actually differ.

I’ve always done collimation of my newts without any binoviewers, reason being is you’re adding additional sources of potential error with the binoviewers. Imagine for example you were using a crooked 1.25” to 2” eyepiece adapter, you wouldn’t want to include that in your collimation. If using binoviewers in the process, which side do you use, left or right? I’ll bet they could give slightly different values.

My thoughts, though admittedly might be entirely wrong, is collimation is to align the optics of the telescope itself. This is what creates the image plane the binoviewers focus on and magnify, etc. While used in conjunction, not sure they are exactly the same system so to speak.
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#7 noisejammer

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Posted 30 November 2021 - 05:26 AM

It may be that the weight of the binoviewer is pulling the focuser draw tube out of alignment. Assuming you focuser is correctly adjusted, there's not much you can do about this because the deflection will vary as you point your scope around the sky.

 

You can test this by installing an eyepiece in the focuser and then pull down on it with about the same torque that the binoviewers generate. If it moves, you need to determine where the motion is - fit of the components, the draw tube, flexing of the upper ring, motion of the truss tubes....


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