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Equipment Discussions >> Cats & Casses

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Simon Alderman
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Reged: 08/22/12

Does "seeing" have an effect on sct collimation?
      #6171210 - 11/01/13 10:49 PM

I was curious if the seeing conditions have an effect on the actual collimation process of an sct. Since the goal is to align the optics, it seems that if the atmosphere isn't as stable or as transparent as one would like that it wouldn't have a great affect. I realize that I wouldn't be able to enjoy the result of a nicely collimated telescope, but I should still be able to work on getting it collimated though, right?

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Eddgie
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Reged: 02/01/06

Re: Does "seeing" have an effect on sct collimation? new [Re: Simon Alderman]
      #6171255 - 11/01/13 11:32 PM

When most people envision collimation, the think of defocusing the telescope and centering the secondary shadow in the Fresnel pattern.

This is at best "Rough" collimation.

Precise collimation is only possible when seeing is very good to excellent.

Precise collimation Is only possible when you can see a first diffraction ring and collimate so that the ring is evenly illuminated all the way around the Airy Disk.

But that is rarely possible to get precise collimation because seeing rarely allows a mostly stable first ring to be visible.

But you can get close with defocus, and the less defocus you use, the closer you can get.

But here is the good news. People talk on the forum like SCT collimation is more critical than collimation for other scopes and this really isn't the case.

It takes about 3 arc minutes of miscollimation before an experienced observer has even a chance of seeing the contrast loss that will result from this.

And 3 arc minutes of miscollimation in a fast reflector will do about the same amount of damage as 3 arc minutes of miscollimation in an SCT,

For any scope that has coma, miscollimation will have the same affect, which is to remove energy from the Airy Disk.

But 3 arc minutes is about what it takes before meaningful damage will occur.

And the good news is that even when seeing is not great, you can often see that this much miscollimation is present simply because the star will appear slightly elongated in one direction even if seeing is not perfect. The energy that is removed from the Airy Disk is thrown to one side of the star.

And when this occurs and you see it, you an often just touch up collimation so that the "Spray" of light randomly dances around the center point. This will get you close.

But perfect collimation requires very good to excellent seeing.

You can often get under three arc minutes though even in less than great seeing by watching the blur to see if it circular.

If seeing is not perfect but there is a wavering ring, you can also get collimation near perfect simply by averaging out the way the ring forms or shimmers.

Truly perfect collimation requires perfect seeing but getting under 3 arc minutes does not, and 3 arc minutes or better and you are getting most of the contrast transfer that the instrument can provide.


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Simon Alderman
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Reged: 08/22/12

Re: Does "seeing" have an effect on sct collimation? new [Re: Eddgie]
      #6171287 - 11/02/13 12:01 AM

Thanks' very much for the explanation. Sounds like "rough collimation" is where I'll start and work my way towards that airy disk.

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WesC
Carpal Tunnel
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Reged: 02/06/13

Loc: La Crescenta, CA
Re: Does "seeing" have an effect on sct collimation? new [Re: Simon Alderman]
      #6171405 - 11/02/13 02:49 AM

That's a great explanation! And yeah, rough is all I've ever been able to get too.

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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Does "seeing" have an effect on sct collimation? new [Re: Eddgie]
      #6171412 - 11/02/13 03:02 AM

Quote:

Precise collimation is only possible when seeing is very good to excellent.




This is simply not true when you use modern methods based on hardware and software. MetaGuide has allowed collimation based on the Airy pattern in mediocre seeing for many years. Times change and techniques change.

There is a subtle issue that you should avoid stars far from the zenith, and should probably use a red or IR filter. This is because atmospheric dispersion will make a star somewhat stretched and look comatic. I am not aware of any description of collimating visually with a real star that made this point - but it is noticeable when using MG because it is able to capture such details in realtime at high resolution while collimating.

Yes - collimating with a star out of focus using the "donut" is only a first stage in collimating, and seeing will limit how well you can collimate in focus with the Airy pattern. But MG and a video camera lets you do that even when the seeing isn't exceptional.

Frank


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HowardK
Pooh-Bah
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Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: freestar8n]
      #6171467 - 11/02/13 05:27 AM

I cannot imagine owning an SCT without using Metaguide to collimate.

But I like things as near to perfect as I can get.

My seeing is always poor.
Metaguide shows me my diffraction ring complete and evenly illuminated.
It's amazing.


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David PavlichAdministrator
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Reged: 05/18/05

Loc: Mandeville, LA USA
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6171580 - 11/02/13 08:48 AM

That's terrific...unless you don't own a video camera.

David


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Eddgie
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Reged: 02/01/06

Re: Does "seeing" have an effect on sct collimation? new [Re: freestar8n]
      #6171670 - 11/02/13 10:10 AM

Yes, of course you are correct.

If the OP has the necessary software and hardware, he can collimate without difficulty even if seeing is less than excellent.

My answer was in the context of what I believed the OP to be using which was naked eye.

Edited by Eddgie (11/02/13 10:11 AM)


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Eric63
professor emeritus


Reged: 06/16/12

Loc: Ottawa, Ontario
Re: Does "seeing" have an effect on sct collimation? new [Re: Eddgie]
      #6171723 - 11/02/13 10:51 AM

Only once did I experience good enough seeing to use 450x on a focused Polaris with my small Mak. I was able to get an unbroken first ring. But the good news is that with a Mak I wont have to do this again for a while.

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HowardK
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Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: David Pavlich]
      #6171749 - 11/02/13 11:16 AM

Quote:

That's terrific...unless you don't own a video camera.

David





Do yourself a favour...buy a cheap webcam...100 dollars.
Metaguide is free.
I'm sure you got a laptop


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Ed Holland
Pooh-Bah


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6171861 - 11/02/13 12:19 PM

Or one can attempt collimation on a pinpoint reflection of the sun from a bright object. In this case the result can be excellent, save for one possible factor... It is unlikely the telescope will be pointed upwards during adjustment, as it would be to view the sky. So, there is the possibility that mirror shift could upset the collimation when the scope is used normally

Still I have had good results with this approach, and consider myself picky about optical alignment

Ed


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ATM57
super member


Reged: 01/01/10

Loc: Tehachapi, CA
Re: Does "seeing" have an effect on sct collimation? new [Re: Simon Alderman]
      #6171987 - 11/02/13 01:37 PM

Here is good information on collimation that will cut through all the opinions and misinformation that tends to come up with this subject.

http://legault.perso.sfr.fr/collim.html

Bad seeing will affect precise collimation but from my experience, miscollimation also increases a telescope's sensitivity to seeing.

Scopejunkie


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Skunky
sage


Reged: 09/16/13

Re: Does "seeing" have an effect on sct collimation? new [Re: freestar8n]
      #6172655 - 11/02/13 09:43 PM

 

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korborh
professor emeritus
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Reged: 01/29/11

Re: Does "seeing" have an effect on sct collimation? new [Re: Skunky]
      #6172802 - 11/02/13 11:22 PM

Hmmm...Metaguide is free.
The Hotech device cannot be the final arbiter of collimation. It does not use the full mirror only points where the laser strikes. Collimation has to be finalized using a real star utilizing the entire aperture or wavefront.
So you always need a star or you don't know your collimation.


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Simon Alderman
member
*****

Reged: 08/22/12

Re: Does "seeing" have an effect on sct collimation? new [Re: korborh]
      #6172826 - 11/02/13 11:40 PM

Some excellent info here. The link to Thierry Legaults' site is very good.
I must amit that I'm new to cats as I've always had dobs and small refractors. I had thought that the collimation process was just one of aligning the optics and wasn't familiar with proceeding to the start test.

All excellent info. Metaguide looks very interesting as well, I'm wondering if I can use my Gopro as the vid cam? I'll have to look for an adapter.


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Skunky
sage


Reged: 09/16/13

Re: Does "seeing" have an effect on sct collimation? new [Re: korborh]
      #6172876 - 11/03/13 12:14 AM

 

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WesC
Carpal Tunnel
*****

Reged: 02/06/13

Loc: La Crescenta, CA
Re: Does "seeing" have an effect on sct collimation? new [Re: Skunky]
      #6173001 - 11/03/13 01:13 AM

Bitter much?

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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Does "seeing" have an effect on sct collimation? new [Re: Skunky]
      #6173007 - 11/03/13 01:25 AM

Umm... The HD whitepaper describes how the telescopes receive the final figuring, assembly, and alignment in the factory. It does not imply at all that the delivered telescope is therefore perfectly collimated, or that the user should emulate that method for best results. The whitepaper is not a user manual.

If you consult the EdgeHD user manual, it describes the importance of collimation, and says:

Quote:

To check the collimation of your telescope you will need a light source. A bright star near the zenith is ideal since there is a minimal amount of atmospheric distortion.




They also say:

Quote:

If seeing (i.e., air steadiness) is turbulent, collimation is difficult to judge. Wait until a better night if it is turbulent or aim to a steadier part of the sky. A steadier part of the sky is judged by steady versus twinkling stars.




MetaGuide lets you accomplish the collimation task recommended by the manual even when the seeing is not in this rare state.

I don't know any amateur or professional telescope that would recommend horizontal collimation over in-situ collimation with a real star. It's a fairly obvious case of using a ground truth measurement for optimization rather than a proxy.

Frank


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RossSackett
Carpal Tunnel
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Reged: 08/17/07

Loc: Memphis, TN
Re: Does "seeing" have an effect on sct collimation? new [Re: freestar8n]
      #6173240 - 11/03/13 08:07 AM

MetaGuide is a great tool. I use it with a "Duncan" mask, and it makes it much easier to see the crossing lines. Being able to integrate over the seeing effects means it greatly simplifies collimation even when the seeing is middling but not perfect.

The CN astrophotography community benefits greatly from the active participation of vendors, both commercial and noncommercial. It makes it easy to get rapid feedback on hardware troubleshooting and software workarounds from those in the best position to know, and has resulted in more than one important update to imaging software.

Edited by RossSackett (11/03/13 11:02 AM)


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DesertRat
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Reged: 06/18/06

Loc: Valley of the Sun
Re: Does "seeing" have an effect on sct collimation? new [Re: Skunky]
      #6173435 - 11/03/13 10:26 AM

The reference to the Hotech device as an alternative to star collimation or that it is some way a competitor to classic star collimation or the video tool Metaguide is invalid.

If you read the Hotech Advanced CT Laser Collimator User’s Manual carefully, you will note near the end a section called "Verifying and Fine Tuning Collimation". How is that performed - a star test!

Also included at the end of the Hotech manual is a section titled "Possible Scenarios where the Laser Collimation Does Not Agree with Star Collimation". The implication is important.

No one is 'bashing' the Hotech device. If followed carefully it does permit a first approximation to collimation in the absence of seeing effects. Its a good tool for alignment - which is not the same thing as collimation.

There are 2 major errors made by beginners in SCT collimation. One is making adjustments when the seeing does not permit making accurate assessments. The second is making corrections larger than are called for. Video tools allow these adjustments to be made in less than ideal seeing.

Glenn


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