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Is this a collimation issue?

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33 replies to this topic

#26 Asbytec

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Posted 20 April 2024 - 08:59 PM

In your images above, note how the view through the collimation cap allows your eye to be closer to the diagonal so the diagonal mirror appears larger than the primary reflection further away at the end of the telescope tube. But it provides a wide white background making it difficult to assess your diagonal position as Don mentioned earlier.

 

The view through the site tube, however, provides a thinner white background making it easier to assess your diagonal position. Unfortunately, you cannot see the entire primary reflection. That is because the Cheshire combo pupil high above the focuser places your eye further from the diagonal mirror, so it appears smaller than the primary reflection.

 

You should be able center your diagonal and align the focuser axis. If the cross hair is on the primary center, the primary reflection will be centered under the focuser. If the diagonal is centered under the focuser, it will be concentric with the focuser and the primary reflection. Often, though, the act of tilting the diagonal to align the cross hair can push the diagonal away from focuser center. So, focus on the axial alignments and seeing the entire primary reflection in the diagonal. 

 

Astigmatism is common with a poorly mounted diagonal. It's a worthwhile modification if it can be done so the diagonal can still be centered under the focuser. I've done it a few times. One time, removing a shroud with a lot of batting forced me to redrill the spider vane mounting holes to move the entire spider toward the primary mirror. Check your spider installation to see if this can be done easily. Edit: If you can move it slightly, it may not be binding and causing astigmatism. 


Edited by Asbytec, 20 April 2024 - 09:14 PM.

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#27 Vic Menard

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Posted 20 April 2024 - 09:24 PM

I tried taking a pic of the secondary but the file size is too big so I'll have to do it again later.

We can work with this.

Look at the annotated image below. 

You need to rotate the secondary mirror (red arrow shows direction--NOT amplitude!) to move the retainer clip (red circle) just a bit until it's on the midline (blue line going top to bottom). With the retainer clip on the midline, you'll need to readjust the secondary mirror tilt to center the primary mirror reflection on the midline, which should put the secondary retainer clip (red circle) more-or-less inline with the lower primary mirror clip (green circle).

Then post a new image of your alignment.

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#28 Asbytec

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Posted 20 April 2024 - 10:01 PM

Yep, a bit of rotation will also cause the reflection of the primary center mark to move onto the vertical blue cross hair without tilting the diagonal away from focuser center. The primary reflection will also move in the same direction and become better centered in the diagonal. The focuser axis and the center mark (axial alignment) along with the diagonal and primary mirror clips will all line up. That's all the OP needs. Easy peasy. You can use the "Cheshire" combo tool cross hair. 


Edited by Asbytec, 20 April 2024 - 10:04 PM.

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#29 gcnadam2906

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Posted 23 April 2024 - 09:53 PM

I had to quit before I wanted to, but here's a picture to make sure I'm in the right track.

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#30 Asbytec

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Posted 23 April 2024 - 10:12 PM

Hard to see on my cell phone, but my impression is it looks very good. If I looked down my Cheshire I'd be happy with that. If the cross hair or laser is on the primary center mark, you're goo there.

Eyeballing it, the dark silhouette offset of the diagonal is almost perfect suggesting good diagonal placement. Being that placement is good, the entire primary reflection is seen in the diagonal almost centered in it as expected.

#31 Starman1

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Posted 24 April 2024 - 12:39 AM

I had to quit before I wanted to, but here's a picture to make sure I'm in the right track.

The white donut center marker should be dead center in the black area.

Also, I can't see any crosshairs in the image.  Are you trying to align the secondary with a collimation cap?



#32 gcnadam2906

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Posted 24 April 2024 - 04:53 AM

The white donut center marker should be dead center in the black area.

Also, I can't see any crosshairs in the image.  Are you trying to align the secondary with a collimation cap?

 

I was using a Cheshire but the picture wasn't coming out, and it was getting late so I took a picture through the collimation cap.  Which white donut are you talking about?  The primary mirror donut in the middle of the collimation cap black spot, or the collimation cap white donut in the secondary mirror?



#33 Starman1

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Posted 24 April 2024 - 08:48 AM

I was using a Cheshire but the picture wasn't coming out, and it was getting late so I took a picture through the collimation cap.  Which white donut are you talking about?  The primary mirror donut in the middle of the collimation cap black spot, or the collimation cap white donut in the secondary mirror?

center marker on the primary.

It should be centered in the small black area.

It's off-center in the image, so the primary isn't yet collimated.



#34 Vic Menard

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Posted 24 April 2024 - 09:24 AM

I had to quit before I wanted to, but here's a picture to make sure I'm in the right track.

You are definitely on the right track--much better! As Don noted above, just a tweak on the primary mirror tilt to center the Cheshire ring (yellow circle) on the primary mirror center marker/donut (small red circle).

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Edited by Vic Menard, 24 April 2024 - 09:27 AM.



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