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Collimation help requested

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#1 Dobs O Fun

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Posted 26 September 2021 - 05:01 PM

All summer seemed to be okay until it got cooler, and perhaps most of all I may have bumped.

 

Sorry ahead of time these photos are not the best quality and the phone wants to focus on the Cheshire cross hairs.

 

It seems to me that I need to bring the stalk on the secondary in a bit and readjust secondary.  

 

Im looking for a second motion or if I'm not looking at this correctly.

 

Thanks again.

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#2 Asbytec

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Posted 26 September 2021 - 05:30 PM

You're secondary might be okay as is, but you can better center it if you want. It's part of the collimation process, but the secondary placement is not axial collimation. Rather it's field illumination as the secondary captures the full converging light cone and sends it up the focuser. Secondary tilt (not its position) aligns the focuser axis to the primary center. You only have a slight rotation "error" as evidenced by the reflection of dark secondary shadow budging toward about 3:30. You can still achieve very good axial collimation. 

 

I understood your question is about axial alignment. For that we need to see the primary center mark (hopefully in better focus) to be under the cross hairs. The cross hair and the Cheshire pupil define the focuser axis. They need to point to the primary center mark, but your image above looks a little off. You're camera lens might not have been centered on the Cheshire pupil, so there may be some parallax error. Since we cannot see the primary center mark, I can't say much about your primary mirror's axial alignment. I do understand sometimes the camera has a mind of its own, but maybe shining some light down the tube will allow the camera to focus better on the primary center and the reflection of the Cheshire ring. 

 

BTW, a Cheshire with cross hairs is called a combo tool, it's a combination of a Cheshire for primary alignment and a site tube for secondary placement and tilt alignment. It looks like you covered the primary reflection in your bottom image, and people do it to ignore all those confusing reflections. But, really, you wanna see those reflections at some point during axial collimation. It's helpful to see them during secondary rotation and refining your collimation at the apex, too. One place a white piece of paper is helpful is on the tube wall behind the secondary and opposite the focuser. It's helps us better see the secondary position. But, for axial collimation, we need to see the primary center mark. 


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#3 ngc7319_20

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Posted 26 September 2021 - 05:38 PM

My impression is that the secondary could be rotated a tiny bit (image looks slightly out-of-round), and moved towards the primary a tiny it, but it is already very close.  The more important adjustments are those of the primary mirror, but I can't tell anything about that from these photos.


Edited by ngc7319_20, 26 September 2021 - 05:38 PM.

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#4 Vic Menard

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Posted 26 September 2021 - 06:25 PM

Hard to tell with the images you've provided.

I can't see the primary mirror center marker in either image, so I have no idea if the primary mirror is centered under the focuser.

You need to make these three circles concentric (or at least pretty close):

the bottom edge of the sight tube,

the actual edge of the secondary mirror , and

the reflected edge of the primary mirror.

 

An 8-inch f/6 is pretty forgiving when it comes to "precision" collimation--I suspect you're already very close (just a couple of minor "tweaks"). What collimating tool are you using? 


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#5 Dobs O Fun

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Posted 26 September 2021 - 07:08 PM

Thanks guys.  My phone definitely has a mind of it's own.

 

I moved the stalk in a bit, readjusted the secondary and have not yet done the primary.  I believe I will do a star test first.

 

Thanks for the feedback.

 

I'm using a celestron Cheshire btw.

 

Attached is the final.  I believe its close.  Still not completely concentric but....

 

 

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#6 Vic Menard

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Posted 26 September 2021 - 08:23 PM

You're pretty close. The green circle is the optimal secondary mirror placement--it looks close enough to me. The yellow circle is the optimal primary mirror tilt adjustment (I'm assuming the primary mirror center marker is under the cross hairs)--looks like you're about 0.15-inch off (read error). The actual error is half the read, so about 0.08-inch. The allowable error (for high magnification performance) is about 0.05-inch for your f/6 primary--so just a little tweak.

 

(The violet circle is the silhouette reflection of the secondary mirror, which, at f/6, should normally show a very slight offset away from the focuser.)

 

 

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#7 Dobs O Fun

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Posted 26 September 2021 - 08:37 PM

Thank you Vic.

You mentioned about the error at high mag. I never thought of that. Planetary views (albeit low angle) at high mag are blurry. I thought it was totally atmospheric. I see what you're saying. Going beyond 9mm in my 1200 has not been that good.

I may make a small tweak then I will do a star test tonight.

What amazes me was that after I loosened up the stalk a bit that as I tightened the secondary down, how it came together without much fuss. I thought I was going to fight this all night.

I'll try a little and update perhaps tomorrow.

Thanks again.

#8 Asbytec

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Posted 26 September 2021 - 09:40 PM

"...and have not yet done the primary. I believe I will do a star test first."

You should collimate the primary axis, first. It is responsible for putting the coma free area in the center of the FOV.


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