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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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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
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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
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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

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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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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
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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
Carpal Tunnel


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
sage


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
scholastic sledgehammer
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Reged: 01/29/11

Loc: Arizona
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
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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
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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
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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
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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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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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HowardK
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Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: Skunky]
      #6173679 - 11/03/13 12:37 PM

Quote:

Quote:



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.

Frank




Another plug, free advertising, from Frank..

You can also use a Hotech Advanced Laser Collimator. And you don't even need a star..





Excuse me skunk spliff......

Frank's Metaguide is FREE ...no plug from him needed as there's nothing in it for him.

Metaguide collimates on an in focus real star with the diffraction ring clearly seen in awful seeing....Pickering 3-4...... Ask me to post an image of the airy disc and diffraction ring from my SCT...


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HowardK
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Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6173695 - 11/03/13 12:53 PM

These forums can really get you down.

Hotech cannot be used for perfect, final collimation.
You have to collimate on an IN FOCUS STAR in the sky with the scope pointing up (primary will tilt) and adjust the secondary on the airy pattern with that DIFFRACTION RING.... there is no other way....if the ring is not whole and evenly illuminated then it is not perfectly collimated.

This is why Hotech tell you to finish the job on a real star.
This is why Celestron say in all their manuals to collimate on a real star in the sky or a sunlight glint on a high pole...as long as the scope is pointing UP say 45 degrees plus.

An SCT new from Celestron or Meade, etc. will have to be unboxed, set up, mounted and finely collimated on a real star high in the sky...if high power planetary or double star observing is your thing.

If the seeing is no good, and it never really is, then the only way is with a cheap webcam, a laptop, Barlow and Frank's most wonderful FREE to all users Metaguide software which will clearly show that illusive diffraction ring surrounding the bright Airy disc... A few tiny tweaks on a collimation screw later and that rough but useable factory new or Hotech adjusted optical instrument will be in spot on collimation on an IN FOCUS STAR.

'Nuff said'


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Skunky
sage


Reged: 09/16/13

Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6173858 - 11/03/13 02:28 PM

 

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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? *DELETED* new [Re: Skunky]
      #6173941 - 11/03/13 03:20 PM

Post deleted by RossSackett

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HowardK
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Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: RossSackett]
      #6173955 - 11/03/13 03:28 PM

I have also seen collimation degrade awfully between horizontal/artificial star and pointed up at 60 degrees.

There's no doubt...fine collimation has to be done pointed up at an in focus star or glint of sunlight.

Some people don't want to accept stuff that they don't know.
I hope skunkman enjoys his views.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174465 - 11/03/13 08:52 PM Attachment (4 downloads)

Quote:

If the seeing is no good, and it never really is...



Well, never say never.

The tropics can put up some excellent seeing in modest apertures, anyway. I can regularly get down to 4 waves and sometimes less than one wave prior to final touch up on the first ring itself. (Had to sketch it one night, seeing was awful good...absolutely awful. I mean, inspiring awful.)

Really, though, it might be 7 of 10 is sufficiently good seeing in-focus with patients, but 8/10 and better allow for very close to focus inspection without the Poisson spot jumping all over the place and the first bright ring closing in on it.

Otherwise I totally agree with you, use a star in focus when seeing permits. If not, nothing wrong with a web cam and some free software (thanks Frank.) There is a certain beauty to a well collimated star image with an equally bright ring and a certain confidence you're getting the most your scope can offer especially when seeing cooperates, too.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174503 - 11/03/13 09:13 PM

Quote:

I have also seen collimation degrade awfully between horizontal/artificial star and pointed up at 60 degrees.



If that was the case then SCTs would not hold collimation on the other side of the meridian. There is definitely something wrong with your OTA. I would call Celestron.


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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: Alph]
      #6174595 - 11/03/13 10:16 PM

"These forums can really get you down."

Well, I agree that it can be hard to read past some of the vitriol apparent in the remarks, but as the OP i feel like my question was answered. Plus I learned a few things I hadn't expected to. I'm looking forward to some good seeing so I can spend some time on dialing in the collimation on my little sct and I may look into metaguide at some point. As the Hotech system costs more than I have in my entire scope, I may hold off on that for a bit...;)

Thanks' to all for the info!


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HowardK
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Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6174796 - 11/04/13 02:27 AM

Quote:

Quote:

I have also seen collimation degrade awfully between horizontal/artificial star and pointed up at 60 degrees.



If that was the case then SCTs would not hold collimation on the other side of the meridian. There is definitely something wrong with your OTA. I would call Celestron.




You are right....without mirror locks collimation is likely to change across the meridian......

That's why celestron now fit solid mirror locks....not 1 but 2 ....to stop the mirror flopping.

Any top planetary imager will test collimation on a star near the planet.
If there is a meridian flip then they will recheck.

For DSO's it's not relevant.
High mag planetary, double star stuff...it is.

I said my bit
If you do not think a large mirror titls on its bed of grease as it swings around the sky then you are entitled to that belief.

Just don't ask me to image Jupiter this season through your scope!
I'll use mine thanku!


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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174798 - 11/04/13 02:29 AM

Guy in a furry hat...

My seeing is never better than 4/10.
I need Metaguide.

Great post.
Lovely sketch.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174824 - 11/04/13 03:26 AM

Howard, do you use larger aperture?

Yea, thank you, I had to sketch it never having really seen so close so clearly before (accustomed to average or lesser seeing most of my life.) It was stunning, one of those nights where you look skyward and offer a silent prayer of thanks. The colors were interesting, too. (Arcturus, IIRC)

But, the whole point being, seeing does affect our effort to collimate, IME. Good seeing allows one to really dial it in and, in conjunction with good seeing, good collimation really boosts the observing experience. If seeing is not as good, then the image is less steady and centering is less certain. Then you can use a tool that captures the disc and rings in real time. You're still optimizing a variable you can control.

Long before software tools were readily available (for SCT mostly), we used to use distant sun lit point sources before the stars came out and even check it after dark. Maybe fiddle with it. I am sure software makes it a snap and is more accurate.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174829 - 11/04/13 03:41 AM

Quote:

If you do not think a large mirror titls on its bed of grease as it swings around the sky then you are entitled to that belief.



As I said call Celstron. Tell them to fix it.


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HowardK
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Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6174867 - 11/04/13 05:08 AM

I called celestron UK today...or the importer should I say.

They told me to collimate on a real star near zenith to avoid mis collimation when the mirror tilts from horizontal.

Just like it says in the celestron manual.
Your scopes have very tight baffles so that your mirror never tilts..you are very lucky....you must have zero focus shift as well and never need to lock down the mirror locks if you have them.

Or maybe you only look at objects below 20 degrees?


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6175277 - 11/04/13 11:34 AM

Quote:

They told me to collimate on a real star near zenith to avoid mis collimation when the mirror tilts from horizontal.



That's right! That means you can also collimate SCT in the horizontal position. Obviously you can't use a real star when the OTA is in the horizontal position. What you can use is an artificial star or the HotTech Collimator. Let's put that urban legend and myth to rest.


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HowardK
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Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6175317 - 11/04/13 12:04 PM

Ok alfie

I give in
You are right
You CAN collimate an SCT horizontally with an artificial star
I have been misinformed
Thanx for your input on this


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WesC
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Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6175382 - 11/04/13 12:41 PM

...in his opinion.

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DesertRat
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Reged: 06/18/06

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Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6175637 - 11/04/13 02:39 PM Attachment (9 downloads)

There is no urban legend concerning horizontal collimation that needs to be put to rest. One can perform a collimation procedure horizontally using an artificial star or using a laser system.

But the acceptable amount of coma and/or astigmatism the intended application can tolerate varies mostly on the image scaling, be it visual or electronic. That (and other points not covered here) will determine the best collimation procedure.

For viewing DSO's at 100-150X one can accept a fair amount of coma without excellent collimation, especially in average or worse seeing.

For high resolution imaging of planets the requirement is pretty severe, meaning 1/14 wv rms of coma or something approaching the classic 'diffraction limited' definition falls well short of the needed accuracy of collimation.

Attached is a 3X IR image of Aldebaran at approx 0.033"/pix taken in mediocre seeing (64 frames of > 2000). It represents the minimum acceptable collimation error I tolerate. The star here needs to be moved about 45 arc sec to the right. Experience has shown that a collimation error greater than this can be damaging in green and blue light, which are much more sensitive, and show coma more dramatically when seeing permits. To overcome dispersion it is essential to collimate with a star as far overhead as practical.

Stars are plentiful and free. Video cameras are inexpensive, and the software tools are free as well.

Glenn


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RossSackett
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Re: Does "seeing" have an effect on sct collimation? new [Re: WesC]
      #6175648 - 11/04/13 02:42 PM

After viewing the HOTECH laser collimation videos I can see why some people are so invested in horizontal collimation--the gear is awfully expensive and the adjustments very involved compared to star collimation. Sure would be a waste if it turns out that collimation shifts with scope elevation, as many of us have experienced. I could be mistaken but I found no mention of turning the focuser knob counterclockwise to seat the mirror or locking the primary before beginning the procedure--you could waste a lot of time making twitchy adjustments if you ignore those steps. Maybe it's a great product and repays the investment; Rod Mollise seems to like them and that says a lot to me. But I've seen apparently well-aligned scopes that were still out of optimal collimation in a star test. Newts are SO much easier...

Edited by RossSackett (11/04/13 03:15 PM)


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Eric63
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Re: Does "seeing" have an effect on sct collimation? new [Re: RossSackett]
      #6175853 - 11/04/13 04:34 PM

You guys just need a Mak!! It will hold collimation from one good seeing period to another while a SCT might lose collimation before another good night of seeing permits a star test. Hotech Lasers and webcams, what are those? Just show me Polaris!

(Sorry I couldn’t resist)


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drollere
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Re: Does "seeing" have an effect on sct collimation? new [Re: DesertRat]
      #6176157 - 11/04/13 07:47 PM

i am also not much of a fan of hotech equipment, and leave it at that. but even with good equipment, the fact that you can see a problem in a star image means that you can also see directly how to fix the problem -- no hardware required.

yes, you can only do approximate collimation in bad seeing, but the bad seeing will make your collimation error inconsequential anyway. you typically will get the accuracy of collimation you can use.

if your scope has a central obstruction, use the extrafocal side of focus that shows you the tiny poisson spot, not the intrafocal side that shows you the black hole in a bagel. the poisson spot, like the airy disk, is surprisingly robust against seeing. in poor seeing the problem is not the center of the defocused image, it's the rapidly moving circumference that makes collimation difficult.

collimation is maintenance, not configuration. you should routinely (like, why not, every night before you start observing) examine a slightly defocused star image. this lets you check the "mirror seeing" or thermal mirror plumes (usually as an apparent diffraction "wedge" arising from the center of the mirror), bad collimation, as well as atmospheric currents.


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azure1961p
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Re: Does "seeing" have an effect on sct collimation? new [Re: drollere]
      #6176252 - 11/04/13 08:40 PM

Exactly Bruce. If the seeing us so poor the star can't be as collimated as a better night - what's the net loss then??? Conversely on the best night with the stilleat air will allow the cleanest diffraction patterns and in the kind of conditions probably needed to see the difference. It kind if takes care of it itself it would seem. Now if the best collimation were oy possible in the worst nights - well then that'd be a problem.

Pete


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DesertRat
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Re: Does "seeing" have an effect on sct collimation? new [Re: azure1961p]
      #6176529 - 11/04/13 11:07 PM

Bruce & Pete,
I agree with you to a point. However in lucky imaging which may occur hours after a collimation check , the seeing may have improved enough that the check or adjustment was useful. Even on a sub par night the seeing may improve for enough moments to be useful in imaging. I almost always check collimation, but rarely make changes unless the configuration has changed.

Back to the OP's question, if I were only doing visual collimation in poor seeing I would not make any corrections, unless the error was huge, like a decentered donut. If it was known to be ok the last time seeing was better, I would not make any corrections.

The benefit of video collimation is that lucky imaging of an airy pattern can facilitate small corrections even in average seeing.

Eric,
I have a IM180 and an Edge14 SCT. Both hold collimation quite well.

Ross ,
SCT collimation is one of the easiest to perform. There are only 3 screws to adjust, and most good SCT's hold collimation well for months. Sometimes the endless threads on collimation in this forum may give people the impression it is a big problem. It is not.

Glenn


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PGW Steve
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Re: Does "seeing" have an effect on sct collimation? new [Re: DesertRat]
      #6176621 - 11/04/13 11:59 PM

There seems to be some firm beliefs in regards to OTA position and mirror flop when it comes to collimation.

I think it is a known fact that there is some degree of primary mirror movement in standard C and M brand SCT's.

My first example will be collimating straight up. The primary mirror should be neutral and sitting as close to its mechanical centre as possible. This will yield collimation at '0'. As I slew around the sky, and my mirror shifts and amount of '1', my optical train will be out by '1' from the value I collimated at.

Now if you collimate in a horizontal position, and slew up to the zenith, you will be out by '1'. Carry on slewing to a direction opposite to where you collimated and you can approach a worst case scenario of '2'....albeit at the horizon where you won't be observing anyway unless you are trying for Omega Centauri from Ottawa Ontario.

Collimating at zenith has less air mass, and less atmospheric affect on seeing, no disputing this. Perhaps even the tube currents at zenith being straight up vs boiling the star image in the tube will be more stable. I've never gone from say 60 degrees to zenith in a period of a minute to verify this, but the science makes sense.

There is a lot of merit to collimating at zenith on an actual star.

I've used CCD Inspector for CCD assisted collimation and I am happy with the results.

Metaguide sounds like a nifty product, does it work with a DMK41??


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freestar8n
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Re: Does "seeing" have an effect on sct collimation? new [Re: azure1961p]
      #6176734 - 11/05/13 02:50 AM

Quote:

If the seeing us so poor the star can't be as collimated as a better night - what's the net loss then???




Seeing changes over time and the situation for collimation is similar to the one for focus. Software can help find true focus for a real star in-situ even when the seeing makes it hard to tell visually, and if you image for some time and the seeing improves, you will reap the benefit of that optimal focus. Similarly, video and software lets you collimate optimally even if the seeing isn't ideal, in-situ using a real star, so that when seeing improves, you "see" it.

If the collimation isn't optimal and you just leave it that way, there will still be nights of different performance where things are better one time than another - but on the really good nights you may be limited by whatever collimation state the 'scope is in - and you won't know it.

Where this really matters is planetary video - and although those guys tend to keep their methods to themselves - I am pretty sure they collimate all the time. With skill, which they appear to have, they may be able to do well just by looking at the fuzz around jupiter's moons, for example, while studying the video and making small adjustments. This is completely in-situ at the precise altitude of the object because it is collimating on the actual object with the actual imaging train.

Frank


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freestar8n
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Re: Does "seeing" have an effect on sct collimation? new [Re: drollere]
      #6176767 - 11/05/13 04:04 AM

Quote:

examine a slightly defocused star image.




A good defocused star image is neither necessary nor sufficient for a good in-focus Airy pattern. Decentering of a spherical secondary is a form of apodization - not aberration.

Frank


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Namlak
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Re: Does "seeing" have an effect on sct collimation? new [Re: freestar8n]
      #6181962 - 11/07/13 10:36 PM

I see this idea that an SCT pointed straight up has it's mirror at the mechanical center when it seems to me that the mirror is being held up by the off-center focusing screw and leaning against the baffle tube, off-center.

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