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Laser collimator or Cheshire or both?


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8 hours ago, Pixies said:

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There is actually a collimation tool that has this kind of concentric ring arrangement built in, no software needed.

That would be the Concenter eyepiece: https://www.firstlightoptics.com/other-collimation-tools/ts-concenter-2-adjustment-eyepiece-for-newtonian-telescopes.html

The view from this towards the secondary looks like so:

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There is no room for guessing, i can immediately see that while its pretty good, its not perfect. Since there are a lot of rings you are guaranteed to have a focuser position where the rings are the exact same size as the edge of the secondary. Very easy tool to use, and most importantly very difficult to misunderstand!

In this example the uncoated rough mirror edges are blackened, so its even easier.

You can also collimate the primary with this and there is no need to take offset or mirror center mark into account since the collimation happens using the outer edge of the mirror.

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Hi Laurance,

I've had two lasers, the cats absolutely love them. One of them literally tried to run up my wall after the dot one night, the other stated doing a moonwalk across the kitchen floor trying to get the dot between her hind paws.

The thing with lasers is that they can't tell if the secondary is rotated correctly. Well that and all the other things such as they need to be sat squarely in the focuser, need collimating themselves, etc.

I recommend a Rigel Aline Collimation Cap and a premium Cheshire sightube.

Be sure to check out and maybe follow this thread also for 130P-DS owners.

Best of luck

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14 hours ago, Spile said:

I also do not recommend “hiding” the reflections in the primary. The sight tube cross hairs need to align with the centre mark and also the “direction” of the offset secondary (r2) should be toward the primary as outlined above.

I know what you mean, but for people new to collimation, the multitude of reflections can be a bit confusing.

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On 28/12/2021 at 13:54, Mr Spock said:

No collimator gets it exactly right. You always need to finish with a star test. I use a laser as it gets it close enough, then finish with a star test.

How do you align the secondary under the focuser?

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8 hours ago, Pitch Black Skies said:

How do you align the secondary under the focuser?

See https://astro.catshill.com/collimation-guide/

 I start with getting offset right so a=b and then correct tilt and rotation.
Recheck the steps 1-3 and repeat the rotation/tilt corrections until the following reflections are as concentric as you can get them:  

1) Bottom edge of the sight tube,
2) the actual edge of the secondary mirror and
3) the reflected edge of the primary mirror.

 

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9 hours ago, Pixies said:

I know what you mean, but for people new to collimation, the multitude of reflections can be a bit confusing.

I know but the offset reflection is a really useful pointer and more importantly without the centre mark being visible you cannot centre the cross hairs.

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2 hours ago, Spile said:

See https://astro.catshill.com/collimation-guide/

 I start with getting offset right so a=b and then correct tilt and rotation.
Recheck the steps 1-3 and repeat the rotation/tilt corrections until the following reflections are as concentric as you can get them:  

1) Bottom edge of the sight tube,
2) the actual edge of the secondary mirror and
3) the reflected edge of the primary mirror.

 

Thanks mate, I'm familiar with the process but I am asking Mr Spock as just wondering how he does it with the laser.

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I aligned the secondary by using a collimation cap. First I placed some card underneath the secondary so I could see the outline clearly. As my secondary (250 Dob) arrived collimated but off axis, I bit the bullet and went for the full lot. I loosened all the secondary screws until it was floppy. I then arranged it roughly by hand so it has the right axial orientation and was positioned under the focuser. Gradually tightening the screws until it was stable, I then adjusted the screws until the secondary was correctly aligned under the focuser, and I could see all three primary clamps equally in the secondary. Once I'd done this I put the laser in; to my surprise it was close, right on the primary doughnut. A small adjustment got it in the centre.

Next was the primary. This was now easy with the laser and just took seconds to do. Easy with a laser because you can tweak the adjustment screws while looking at the laser from a distance - can't do that with the other methods. Then it was a case of getting outside and doing a star test. At this point only tiny turns of the primary adjustment screws are needed.

 

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