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Help with Collimation

Beginner Collimation
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#1 ctsufer31

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Posted 12 April 2024 - 05:51 PM

I am attempting to collimate my Skywatcher 250p dobsonian telescope.

20240412_col_.jpg

 

This is an image of my mirrors.

Can someone offer some advice on what mirrors I need to adjust?

Thanks.

 

Stephen



#2 Asbytec

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Posted 12 April 2024 - 07:06 PM

The most obvious thing is the black donut (primary center mark) needs to move onto the black dot (collimation cap pupil). You do this by tilting the primary mirror, so the primary mirror optical axis moves to the pupil of the collimation cap. This is the last step in collimation. 

 

I do not see enough information about the alignment of the focuser axis to the primary center. This is the second step in collimation. Do you have any other tools for aligning the focuser axis such as a site tube or a laser?

 

Also, when you take a picture, put a white piece of paper in the tube opposite the focuser behind the diagonal mirror. Take a wider shot so we can see the bottom of the focuser. This helps us see the position of the diagonal mirror. 


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#3 sevenofnine

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Posted 13 April 2024 - 10:47 AM

This video on collimating a Dob helped me a lot. It's pretty funny too. Good luck! borg.gif  

 

https://www.youtube....G98RTP6jbY&t=7s.


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#4 Vic Menard

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Posted 13 April 2024 - 11:10 AM

While I agree with Norme's "most obvious" misalignment, there appears to be another potentially significant misalignment (the red cross hairs indicate the center of the primary mirror).

 

I pushed the exposure and shadow detail to reveal the actual edge of the secondary mirror (green circle). Unfortunately, I still can't see the bottom edge of the focuser drawtube in your image, so there's no reference for the focuser axis (as Norme also noted).

 

I'm also interested in your "collimation cap" as it appears to have a mirror surface--is this an autocollimator? If so, who made it?

 

(Edit to add annotated image.)

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Edited by Vic Menard, 13 April 2024 - 08:48 PM.

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#5 Spile

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Posted 13 April 2024 - 12:49 PM

You need to get the black donut centred over the black dot, but…

before you do that ( by tilting the primary mirror) run through the steps below to ensure your secondary mirror is centred and aligned.
https://astro.catshi...limation-guide/
You will be getting the same advice from everyone here, just follow the poster that is making sense to you as we are all saying the same thing. :-)


Edited by Spile, 13 April 2024 - 12:50 PM.


#6 Asbytec

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Posted 13 April 2024 - 07:47 PM

...there appears to be another potentially significant misalignment (the reed cross hairs indicate the center of the primary mirror).

 

 

I'm also interested in your "collimation cap" as it appears to have a mirror surface--is this an autocollimator? If so, who made it?

 

At first, I wondered why Vic put the red circle and cross hair on the primary mirror instead of the focuser. The obvious answer is we cannot see the focuser draw tube. Then it dawned on me, if and when the focuser is aligned to the primary center the red cross hair would be on the black donut because the primary reflection would be concentric with the focuser draw tube. Even if we could not see the focuser, we know that to be the case. So, there is likely a problem with the focuser alignment, too. It's not obvious if you don't think about it, but ingenius if you do. lol.gif


Edited by Asbytec, 13 April 2024 - 07:52 PM.


#7 ctsufer31

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Posted 15 April 2024 - 12:42 PM

I decided to reshoot the images from my telescope.

This image is through the astrosystems autocollimator:

20240415_auto_.jpg

 

 

 

 

 

 



#8 ctsufer31

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Posted 15 April 2024 - 12:44 PM

Also, this image is through the farpoint astro cheshire:

20240415_cheshire_.jpg



#9 ctsufer31

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Posted 15 April 2024 - 01:17 PM

This video on collimating a Dob helped me a lot. It's pretty funny too. Good luck! borg.gif  

 

https://www.youtube....G98RTP6jbY&t=7s.

Thank you for the link.  I have two laser collimators. I purchased a SVBONY Red Laser Collimator.  Unfortunately, it wasn't aligned. I attempted to align the laser but it didn't work.  I then invested in a Farpoint Astro laser collimator (2 inch).  When I received it, I checked to see if it was aligned.  I found out that this laser collimator was out of alignment.  I attempted to contact Farpoit Astro (by email and a phone call).  They have yet to return my call for support.  Farpoint Astro states in one of their youtube videos that they will realign their laser collimators; but since they are not responding to my phone calls, I doubt it.

 

If you have any experience with Farpoint Astro, let me know what I should do. 



#10 Vic Menard

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Posted 15 April 2024 - 03:30 PM

Also, this image is through the farpoint astro cheshire:

attachicon.gif 20240415_cheshire_.jpg

You're pretty close. Your secondary (green circle) is almost centered and your secondary mirror tilt is within the allowable high magnification tolerance. If you can adjust the primary mirror tilt to center the primary mirror center marker (small red circle) in the Cheshire ring (yellow circles), your collimation should deliver good image performance. 

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#11 ctsufer31

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Posted 17 April 2024 - 01:23 PM

You're pretty close. Your secondary (green circle) is almost centered and your secondary mirror tilt is within the allowable high magnification tolerance. If you can adjust the primary mirror tilt to center the primary mirror center marker (small red circle) in the Cheshire ring (yellow circles), your collimation should deliver good image performance. 

Thanks for your help and insight Mr. Menard.



#12 Asbytec

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Posted 17 April 2024 - 04:11 PM

In Vic's image above, if you rotate the diagonal slightly the reflection of the primary and the center mark will move upward onto the cross hair. I believe the rotation will be clockwise as seen looking down the telescope tube. Your diagonal is well placed under the focuser (green circle), all it needs is a tiny bit of rotation.

 

Doing so will also center the primary reflection in the already well centered diagonal. Then, the three collimation signatures (focuser draw tube, diagonal, and primary reflection) will be concentric with each other, and the focuser axis (cross hair) will be aligned to the primary center mark. 



#13 Vic Menard

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Posted 17 April 2024 - 06:00 PM

In Vic's image above, if you rotate the diagonal slightly the reflection of the primary and the center mark will move upward onto the cross hair. I believe the rotation will be clockwise as seen looking down the telescope tube. Your diagonal is well placed under the focuser (green circle), all it needs is a tiny bit of rotation.

 

Doing so will also center the primary reflection in the already well centered diagonal. Then, the three collimation signatures (focuser draw tube, diagonal, and primary reflection) will be concentric with each other, and the focuser axis (cross hair) will be aligned to the primary center mark. 

I wouldn't suggest rotating the secondary mirror unless ctsufer31 has a good sight tube and/or a good simple thin beam laser. Without at least one of those tools he'll have to rely on keeping the loose secondary mirror centered while he tries to center the primary mirror reflection, and that rarely happens without a tool that shows the actual secondary mirror centering. If he had Jason's milk jug washer upgrade already installed, he might be able to barely loosen the center screw to enable rotation without impacting the secondary mirror centering. 

 

That's why I suggested getting the primary mirror tilt sorted and using the current secondary placement/tilt "as is". The next critical step would be removing the primary mirror to fix the uncentered donut, ideally with a good reflective donut (Rigel) or triangle (Catseye) that would work with his autocollimator (assuming the AstroSystem autocollimator is a "good" autocollimator) and his Farpoint Cheshire. As it stands, using the current donut/Cheshire alignment should (eventually) be verified with a star alignment.



#14 Asbytec

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Posted 17 April 2024 - 07:05 PM

The center mark is significantly off? Do you evaluate his collimation to be within tolerance? Maybe close enough? 

 

FarPoint usually provides very good factory collimation of their lasers, but he says it's off. That's a shame. Mine is dead on. I don't want to doubt the OP, but maybe it's worth another check to test the laser collimation. The laser can appear mis collimated if it does not seat well in the focuser while rotating it. 

 

I don't loosen the center screw to rotate the diagonal. I loosen the tilt screw in line with the focuser just enough so the diagonal will rotate. Then tighten that screw to hold it. It doesn't push the diagonal away from focuser center. I guess there's no guarantee it will tilt back to the center mark without a collimated laser. 


Edited by Asbytec, 17 April 2024 - 07:51 PM.


#15 ctsufer31

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Posted 19 April 2024 - 09:01 AM

The center mark is significantly off? Do you evaluate his collimation to be within tolerance? Maybe close enough? 

 

FarPoint usually provides very good factory collimation of their lasers, but he says it's off. That's a shame. Mine is dead on. I don't want to doubt the OP, but maybe it's worth another check to test the laser collimation. The laser can appear mis collimated if it does not seat well in the focuser while rotating it. 

 

I don't loosen the center screw to rotate the diagonal. I loosen the tilt screw in line with the focuser just enough so the diagonal will rotate. Then tighten that screw to hold it. It doesn't push the diagonal away from focuser center. I guess there's no guarantee it will tilt back to the center mark without a collimated laser. 

I glad to see that your laser collimator is functioning correctly.  My laser collimator isn't.  

Here is a video of my laser collimator as I rotate it in the focuser: https://imgur.com/a/2R9vFGG.

Also, according to a youtube video posted by Farpoint (https://youtu.be/NEQ...MPXJi5140KCZrct), you are not suppose to tighten the screws to hold the collimator.

 

One more thing.  There is a thread in the equipment forum called "Farpoint Astro a good company?".  Some of the other Cloudy Nights forum members disagree with your opinion.  Maybe at one time they were good; but, not anymore.  If you still have doubts, call them on their support number or email them with a support question and see if they return your message?

 

I doubt they will contact you back.


Edited by ctsufer31, 19 April 2024 - 09:25 AM.

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#16 Asbytec

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Posted 19 April 2024 - 09:22 AM

I am sorry your FarPoint laser is not well collimated. FarPoint makes good stuff, but I have had little to no responses from their customer service. I had one defective Cheshire, and one incorrect order that was replaced. So, yea, I am aware of it, and I believe I posted in that thread. Or another one. So, my opinion is mixed. I was fortunate to get everything correct even though they never notified me they were doing it. 

 

My comment above was, and I may have been premature, hoping you held the laser firmly against the focuser when testing it. If the fit is sloppy, of course the laser dot will wonder around. So, I was hoping against hope that was the problem with your FarPoint Laser, and to be sure you understood how to do the test. It appears you do. I watched the video. Starman1 replied to your post in the thread on how to align a laser. 

 

Edit: Just saw the video of the laser test. Yea, that's not good. 


Edited by Asbytec, 19 April 2024 - 05:35 PM.


#17 Vic Menard

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Posted 19 April 2024 - 09:22 AM

The center mark is significantly off?

The red cross hairs indicate the actual (measured) primary mirror center. From what I could find online, the secondary mirror minor axis is 64mm, which means the primary mirror center marker/donut diameter is ~8mm, and it's decentered by ~4mm. The allowable error for secondary mirror tilt (10-inch aperture without coma correction) is ~8mm. The allowable Cheshire read error for primary mirror tilt (for an f/5 Newtonian) is ~1.4mm.

 

(Considering the actual center of the primary mirror is near the bottom of the primary mirror center marker/donut, the alignment in post #10 should probably be left "as is". But I still recommend assessing/verifying the primary mirror tilt alignment using Mike Lockwood's method which can be found here:  http://www.loptics.c.../starshape.html  )

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Edited by Vic Menard, 19 April 2024 - 12:03 PM.

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