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Collimation with a laser on a Dob

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#1 SatGod

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Posted 19 January 2022 - 05:05 PM

So I am returning to the hobby for the first time since I was a teen. Picked up an 8' Dob and needed to collimate after it was shipped to me by another CN member. I chose a laser and it appeared to work fine. However, I was unable to achieve focus on the moon with an EP. Tried collimation again, ultimately same result. 

 

My next step was to purchase a 2nd laser and a Chesire and low and behold, the original laser is definitely off as the 2nd laser and the Chesire give me a matching result and the 1st laser does not. I will return it. I will wait for a clear night to verify I can now achieve focus. 

 

Just a heads up to others who rely on the laser that it is not always accurate and not necessarily the best tool for the job.

 

On another note, anyone have a good procedure to collimate a laser collimation tool?   



#2 vtornado

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Posted 19 January 2022 - 05:21 PM

To adjust the primarmy

 

http://www.smartavtweaks.com/RVBL.html

 

This will remove slight laser mis collimation, and laser registration issues.

 

On an f/6 scope you can get quite good results with a collimation cap.



#3 photoracer18

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Posted 19 January 2022 - 05:24 PM

Collimation has nothing really to do with the ability to focus. Even bad collimation will get to focus, just the image won't be good. Unless the optics are so far out of adjustment the light is not getting to the eyepiece at all. If you take the eyepiece out of the holder and point the telescope at the moon you should see an image of the moon looking down the eyepiece drawtube in the center of the secondary mirror. If not, then you need a Chesire eyepiece.


Edited by photoracer18, 19 January 2022 - 05:25 PM.

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#4 photoracer18

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Posted 19 January 2022 - 05:29 PM

So I am returning to the hobby for the first time since I was a teen. Picked up an 8' Dob and needed to collimate after it was shipped to me by another CN member. I chose a laser and it appeared to work fine. However, I was unable to achieve focus on the moon with an EP. Tried collimation again, ultimately same result. 

 

My next step was to purchase a 2nd laser and a Chesire and low and behold, the original laser is definitely off as the 2nd laser and the Chesire give me a matching result and the 1st laser does not. I will return it. I will wait for a clear night to verify I can now achieve focus. 

 

Just a heads up to others who rely on the laser that it is not always accurate and not necessarily the best tool for the job.

 

On another note, anyone have a good procedure to collimate a laser collimation tool?   

Yes I can tell you how to collimate a laser. And yes a laser is not the final test, a star test is.



#5 Daveatvt01

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Posted 19 January 2022 - 05:57 PM

On my 8” f5.9 Orion dob some eyepieces will not focus if fully seated in the focuser. They need to be pulled out a little bit further away from the primary, maybe 1/8” or 1/4”? I’m not sure if the ES scope is the same, but it’s easy to try. If focus is your issue.


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#6 photoracer18

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Posted 19 January 2022 - 06:03 PM

On my 8” f5.9 Orion dob some eyepieces will not focus if fully seated in the focuser. They need to be pulled out a little bit further away from the primary, maybe 1/8” or 1/4”? I’m not sure if the ES scope is the same, but it’s easy to try. If focus is your issue.

To check your scope if you are all the way out on the focuser and it does not focus slowly pull it back out with your eye on it and see if it focuses someplace farther back. If so calculate how far and then buy an extension tube slightly longer than you need. Also check any other eyepieces you have.

 Its also possible that an eyepiece can't go in far enough to focus. Only fix for this is too move the mirror farther down the tube. One way is to adjust the collimation of the primary with the adjusting screws as far forward in the tube as possible. If you want to do this first see how much unused length on the screws you have to play with. Generally the focus range of a Newt is fairly short, so some small adjustments in the mirror position can yield enough extra travel to often make ones needing in travel to focus even if you gain as little as 1/4".


Edited by photoracer18, 19 January 2022 - 06:09 PM.


#7 Bean614

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Posted 19 January 2022 - 06:38 PM

If you have an Owner's Manual,  or if not,  locate it online, for that scope.  You never did say what the focal length was of the eyepiece you used, nor whether it was 2 inch or 1.25 inch.  This is important! Most of the Synta (Orion, Skywatcher)Dobs came with screw-in extension eyepiece holders.  If you bought this used, you might not have received one. Anyway, read the Manual,  it's important.   Additionally,  try, with whatever eyepiece you were using, to move just the eyepiece in or out of the focuser tube, after you've gotten as close as you can to focus. I really don't think collimation is the issue here.



#8 MellonLake

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Posted 19 January 2022 - 07:03 PM

Collimation is 3 Steps:

 

1) Using a sight tube centre and round the secondary.  This is best done with a sight tube or Cheshire.  Looking at the outer diameter of the secondary mirror and make sure it is concentric with the inner diameter of the sight tube.  This ensures the focuser is looking at the secondary mirror.  You really don't need to worry to much about centring at this point as it is really a more advanced step and can cause beginners frustration.  If the mirror is not centred it just means the field of view will not be uniformly illuminated which our eyes are not sensitive to.  

2) Using the laser or cross-hairs on the Cheshire (if it is a combo tool). Align to the centre of the mirror marker.  This ensures the secondary mirror is looking at the primary mirror.

3) Using the Cheshire (which will be better than the laser for this step) Align the black dot to the centre of the mirror marker (or the mirror marker in the Cheshire ring if it is a true Cheshire).  This ensure the primary mirror is looking at the secondary mirror.

 

These steps must be done in order always ending on Step #3



#9 briansalomon1

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Posted 20 January 2022 - 10:53 AM

Here's a link to a demonstration for how to align a laser collimator.

 

https://www.dropbox..../Align.mp4?dl=0



#10 Frugal Astronomer

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Posted 22 January 2022 - 02:32 PM

As a former owner of an 8" dob, your problem definitely sounds like a focus problem (probably needed an extension tube for the eyepiece you were using) rather than a collimation problem.  Your collimation would need to be pretty bad on an f/6 to not get the moon in focus. 



#11 Spile

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Posted 22 January 2022 - 04:49 PM

Do feel free to give my guide a spin https://astro.catshi...limation-guide 



#12 Chris Cook

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Posted 25 January 2022 - 10:22 AM

Do feel free to give my guide a spin https://astro.catshi...limation-guide 

This is one of the best collimation guides I've seen.  I've printed it out to have as a reference.  Well done!


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#13 SatGod

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Posted 27 January 2022 - 05:20 PM

Thank you all for your input. I was not able to achieve focus after re-collimating. I have caught up on the posts here. Now will try those suggestions and look over the attachments added in the posts. 




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