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Laser Collimator
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   General Equipment -> Collimation AidsMessage format
 
mkjt88
Posted 9/14/2008 11:53 AM (#9154)
Subject: Laser Collimator



Veteran Junky

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Location: Illinois		I was searching for a laser collimator as my dob came with a cap collimator which I don't even care to own. All of them were at least $50-60+ and now Orion has on on sale for $15 but I have a quick question on how it works. http://www.telescope.com/control/product/~category_id=101607clearan...
Everyone I saw had a way to view the laser on a bulls eye sticker like http://www.opticsplanet.net/zhumell-laser-collimator.html and Orions upgrade of this one, http://www.telescope.com/control/product/~category_id=scope_tune-up...

Now the one on sale for $15 the laser goes straight into the focuser but where does the laser come back to view if it's collimated correctly? It has no place to show the light to line back up with anything.. what am I missing here?
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David Pavlich
Posted 9/14/2008 12:44 PM (#9159 - in reply to #9154)
Subject: RE: Laser Collimator



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Location: Mandeville, LA USA 30.38 X 90.07		I wouldn't mess with the $15 laser. Not worth the money. As a matter of fact, before you get a laser, I'd get the following:

http://www.telescope.com/control/product/~category_id=scope_tune-up...

At f6, the XT8 is not difficult to collimate. The laser doesn't help you to align the secondary under the focuser, the Cheshire does.

And the cheshire can get your collimation very close.

Having said all of that, when I had a Newt, I had the Cheshire, a laser and an autocollimator...a little overkill maybe, but I'm here to tell you that my collimation was really close.

David
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mkjt88
Posted 9/14/2008 1:05 PM (#9162 - in reply to #9159)
Subject: Re: Laser Collimator



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Location: Illinois		Alright, thanks.
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foxfire
Posted 11/6/2011 7:47 PM (#46318 - in reply to #9159)
Subject: RE: Laser Collimator



Extreme Junky

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Location: Mojave Desert, California
HEY DAVID,

I've owned eleven Dobsonian telescopes since 1998, from 12" to 20" in aperture. My current Dob is a 16" f/4.5 Meade LightBridge.

I've always used laser collimators, and I'm currently using a Howie Glatter single-beam unit, which has given me good results over the 8 years that I've owned it.

I read something once about reflector collimation that has puzzled me, and I've been using reflectors for about 36 years.

It was written that some manufacturers of large, fast Dobs will PURPOSELY OFFSET the secondary mirror under the focuser for some technical reason that escapes me.

This would show a positioning error under a Cheshire examination, yet it was purposely installed that way!

Have you or anyone ever heard of this?

Thanks,
HAL






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JohnM
Posted 11/6/2011 8:24 PM (#46320 - in reply to #46318)
Subject: RE: Laser Collimator



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Location: Wyoming Wind River Range		Offsetting secondaries is a whole sub culture.
People who get into it will agonize for days doing calculations for the off set.
It's bizarrely complex and really of no use in the normal world.
During a regular collimation a certain amount of off set occurs during secondary adjustment anyway.
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AlBoning
Posted 11/6/2011 10:55 PM (#46322 - in reply to #46318)
Subject: RE: Laser Collimator


Veteran Junky

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Location: Gainesville, FL		The secondary mirror of my newt is so obviously off center on its holder that one can see that it is. I measured it and compared it to the calculated value using the formula offset = (minor axis/(4*focal ratio) which in turn had to be multiplied by the square root of 2. The difference was less than half a millimeter. I must be unique among newt owners in that my first encounter with offset was a technical discussion about collimation and as a result I accept offset for what it is, the only correct way to position a secondary mirror, that is to say with the optical center of the secondary mirror at the intersection of the optical axis of the primary mirror with the optical axis of the eyepiece.
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JohnM
Posted 11/7/2011 7:16 AM (#46339 - in reply to #46322)
Subject: RE: Laser Collimator



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Location: Wyoming Wind River Range		If you have a visible offset to your secondary mirror it is there purely by accident.
No company making budget priced production instruments does the calculations or makes the critical mechanical positioning needed
to employ offset.

Here's a fairly short page of common collimating myths.
I suggest reading the paragraph about offset particularly.
http://web.telia.com/~u41105032/myths/myths.htm

Offsetting is purely an exercise until you get into extremely fast mirrors and precisely sized secondaries, especially if working toward absolute minimum sizes to reduce CO.
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