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Laser barlowed question

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#1 miguel gonzalez

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Posted 19 July 2021 - 11:01 PM

Hi all...

I have a question about laser barlowed:

 

Any non-aligned laser (internally miscollimated) can do a correct and exact job in aligning the primary?

 

Thanks


Edited by miguel gonzalez, 19 July 2021 - 11:08 PM.


#2 vtornado

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Posted 19 July 2021 - 11:40 PM

I believe the system works with a slightly misaligned laser.

One can rock the barlowed laser in the focuser and the doughnut shadow does not move,

which I think??? is equivalent to the laser being slightly off.

 

Someone correct be if I am wrong.   After I use this method, a star test shows I am right on or very close.

Also agrees with a collimation cap, but that only works with a bright light.

 

My fastest scope is an f/5


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#3 ngc7319_20

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Posted 19 July 2021 - 11:52 PM

Yes, within reason, the laser can be mis-collimated.  If the laser collimation is very far off, and the Barlowed beam misses the center of the primary mirror, then you will have a problem.


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#4 Redbetter

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Posted 20 July 2021 - 01:09 AM



I believe the system works with a slightly misaligned laser.

One can rock the barlowed laser in the focuser and the doughnut shadow does not move,

which I think??? is equivalent to the laser being slightly off.

Rocking the Barlow shifts the center of the Barlow somewhat (which is where the return is being evaluated)--so that produces a small collimation shift. This registration error is small, but the result is that the primary is not aiming quite at the center of the field.  [I suspect this might matter more for coma correctors than without them, since centering of the light cone in the corrector should be more critical to their performance.]  

 

I see this with the 2" Glatter/Tublug in the 20".  The long Tublug with the mass of the laser can make this somewhat sensitive to registration in the focuser.  What happens is that after tightly centering the secondary alignment, I put in the Tublug and despite my best efforts, I often find it is slightly askew on the first pass.  I can adjust the primary collimation, but will see the return diffraction rings are not concentric.  Loosening the screws on the focuser ever so slightly, I can make the Tublug flush, and carefully tighten down again, knowing that I have it when the return rings are well centered.  A small final touch up is needed on collimation after that, because the original return point that was collimated to was slightly off. 



#5 Jon Isaacs

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Posted 21 July 2021 - 03:36 PM

Rocking the Barlow shifts the center of the Barlow somewhat (which is where the return is being evaluated)--so that produces a small collimation shift

 

 

The question here is whether the laser itself can be misaligned.  If you rock the laser in the Barlow, then the shadow does not move.  As ngc7319_20 said, as long as the diffuse laser beam covers the center marker on the primary mirror, you should be good.

 

Jon



#6 Tom Stock

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Posted 21 July 2021 - 04:03 PM

I experimented with this last night and wow.  I wish I'd tried this sooner. 


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