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Equipment Discussions >> Reflectors

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Jason D
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Reged: 10/21/06

Loc: California
Re: New Perspefctives on Newtonian Collimation new [Re: precaud]
      #5679919 - 02/14/13 05:23 PM

Quote:

That I'm aware of, the AS autocollimator is the only game in town for 1.25" . Please correct me if I'm wrong.




There is also 1.25" JSP Easy Tester autocollimator which is the lowest quality autocollimator in the market.


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Perspefctives on Newtonian Collimation new [Re: precaud]
      #5679927 - 02/14/13 05:30 PM

I suppose, then, as with many astronomy products, you have the luck of the draw whether or not you will receive an excellent, good or mediocre sample. Just as we should star test telescopes to determine their quality, we should also test our collimation tools. On the other hand, throwing more money at a problem is no guaranteed method of solving it.

Mike


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Jason D
Postmaster
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Reged: 10/21/06

Loc: California
Re: New Perspefctives on Newtonian Collimation new [Re: Sarkikos]
      #5679972 - 02/14/13 06:04 PM

Quote:

we should also test our collimation tools.



Absolutely.

Quote:

On the other hand, throwing more money at a problem is no guaranteed method of solving it.



That is why everyone needs to do their own research and decide how to best spend their money.


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: New Perspefctives on Newtonian Collimation new [Re: Sarkikos]
      #5679978 - 02/14/13 06:07 PM

Quote:

There is also 1.25" JSP Easy Tester autocollimator which is the lowest quality autocollimator in the market.



Yes, but, as you well know, it's not really worth mentioning.

Quote:

I suppose, then, as with many astronomy products, you have the luck of the draw whether or not you will receive an excellent, good or mediocre sample.



Absolutely... test and verify everything we must, as best we can. But the difference here is, this is a collimation tool, with pretty clear performance expectations based on its description. The way the AstroSystems autocollimator is designed, the alignment accuracy is accomplished by the adjustment of the three screws on the top. Theoretically, every one of them could come out properly adjusted...


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Atl
professor emeritus
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Reged: 04/13/12

Re: New Perspefctives on Newtonian Collimation new [Re: Sarkikos]
      #5680189 - 02/14/13 08:43 PM

Quote:

I think an autocollimator will produce a much closer collimation than a laser. I'd place the laser collimators at about the same level of accuracy as a Cheshire/sight-tube, and even then, only if the laser itself has been collimated.




I meant using a sight tube/cheshire/ autocollimator, and just using a laser to touch it up in the dark.


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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: New Perspefctives on Newtonian Collimation new [Re: Atl]
      #5680281 - 02/14/13 09:54 PM

The manufacturer's response:
"The mirrors in our autocollimators are aligned optically on a 100 foot path, giving 2-3 arc minute accuracy. I don't believe that the accuracy of the ID of a drawtube is accurate enough to see any deviation."

So what exactly is being said here? That, no matter what the tool's rotated orientation, if the reflections stack up, that's good enough and any better than that is not visible? And it doesn't matter if another rotation unstacks the reflections?

Is there any data that supports or refutes his statememt?


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Sarkikos
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Perspefctives on Newtonian Collimation new [Re: Atl]
      #5680322 - 02/14/13 10:19 PM

Quote:

Quote:

I think an autocollimator will produce a much closer collimation than a laser. I'd place the laser collimators at about the same level of accuracy as a Cheshire/sight-tube, and even then, only if the laser itself has been collimated.




I meant using a sight tube/cheshire/ autocollimator, and just using a laser to touch it up in the dark.




Yes, I know, but I would be concerned that the laser could mess up the collimation worse than a slight misalignment that the telescope might suffer in transit to the site. I'd rather take my chances with the slight misalignment.

Mike


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Atl
professor emeritus
*****

Reged: 04/13/12

Re: New Perspefctives on Newtonian Collimation new [Re: Sarkikos]
      #5680330 - 02/14/13 10:25 PM

Recently I have compared my laser and Cheshire and they are the same accuracy wise. I admit my skill with the autocollimator is just beginning to show.

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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: New Perspefctives on Newtonian Collimation new [Re: Atl]
      #5680566 - 02/15/13 01:53 AM

Accuracy:
1.Sight Tube = collimated laser
2.Cheshire = Barlowed laser = Krupa Collimator
3.Autocollimator = class by itself and will collimate to a higher degree of accuracy than #1 or #2. Where this tool's accuracy is paramount is a. if you use a coma corrector in the system, or b. you are trying to design or re-engineer a telescope to be capable of holding collimation as it moves up and down.


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Perspefctives on Newtonian Collimation new [Re: Starman1]
      #5680953 - 02/15/13 09:23 AM



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precaud
scholastic sledgehammer
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: New Perspefctives on Newtonian Collimation new [Re: Sarkikos]
      #5681380 - 02/15/13 12:42 PM

I want to continue to explore this matter, but I'll do it in a new thread.

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Vic Menard
Post Laureate
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Reged: 07/21/04

Loc: Bradenton, FL
Re: New Perspectives on Newtonian Collimation new [Re: precaud]
      #5681461 - 02/15/13 01:13 PM

Quote:

The manufacturer's response:
"The mirrors in our autocollimators are aligned optically on a 100 foot path, giving 2-3 arc minute accuracy. I don't believe that the accuracy of the ID of a drawtube is accurate enough to see any deviation."



I don't find it too difficult to correct the focuser axial alignment to an accuracy of +/-0.02-inch using a Glatter laser with my 88-inch focal length Dobsonian. This reflects an accuracy a bit less than one arc minute.

This same error will cause the P-3 alignment (after CDP) in the central pupil to unstack 0.04-inch, which is quite obvious. A 3 arc minute error would cause almost 4X the error.

Quote:

So what exactly is being said here? That, no matter what the tool's rotated orientation, if the reflections stack up, that's good enough and any better than that is not visible? And it doesn't matter if another rotation unstacks the reflections?



I think what's being said is that as long as the unstacked error (through a full 360-degree rotation) is less than the axial alignment tolerance, the tool is useful. If there's no observable unstacking error, then the tool's internal alignment and registration is essentially transparent--any error you see is attributable to the focuser and/or primary mirror axes.

Quote:

Is there any data that supports or refutes his statement?



There's good documentation on the axial alignment tolerances. These tolerances vary according to aperture, focal ratio and application.


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: New Perspectives on Newtonian Collimation new [Re: Starman1]
      #5681528 - 02/15/13 01:38 PM

Quote:

Accuracy:
1.Sight Tube = collimated laser



I'll assume we're discussing axial alignment (not secondary mirror placement). Then, the sight tube cross hairs aligned to the primary mirror center spot is equivalent to the collimated thin beam laser aligned to the primary mirror center spot. But if you include readability, I would argue that with a thin beam laser I can achieve a focuser axial alignment correction of +/-0.02-inch, about twice the accuracy I can achieve using the sight tube cross hairs. But perhaps that's just me.

Quote:

2.Cheshire = Barlowed laser = Krupa Collimator



Pretty much true as long as the Cheshire pupil is kept small, otherwise, parallax can complicate the read. The Barlowed laser and Krupa collimator are both parallax free. Nils Olof's balanced Barlowed laser is more insensitive to residual focuser axial tilt error than the other tools.

Quote:

3.Autocollimator = class by itself and will collimate to a higher degree of accuracy than #1 or #2.



The single pupil autocollimators can show residual focuser axial errors magnified 2X (P-3 with a CDP), that's better than #1. But stacking by itself does not necessarily indicate a better primary mirror axial alignment than #2 (P-1 stacks when the axes intersect at the COC, and 2-3 disappear when P-1 are stacked). A two pupil autocollimator improves the read accuracy by providing a very precise assessment of parallelism, but for simple, direct interpretation, I still prefer #2.

Quote:

Where this tool's accuracy is paramount is a. if you use a coma corrector in the system...



Which puts real demands on the axial alignments...

Quote:

...or b. you are trying to design or re-engineer a telescope to be capable of holding collimation as it moves up and down.



Depending on the axial tolerances, a good laser might work as well, but if the tolerances are particularly stringent, a good autocollimator is likely to be your best bet.


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graffias79
sage


Reged: 10/01/10

Loc: Edgerton, WI
Re: New Perspectives on Newtonian Collimation new [Re: hottr6]
      #5683476 - 02/16/13 12:56 PM

Quote:

Quote:

May I ask where you bought the book from?



http://bit.ly/14uVf2F





Congratulations, that's probably the most sarcastic thing I've ever seen on the internet. And that's saying something!


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: New Perspectives on Newtonian Collimation new [Re: graffias79]
      #5683513 - 02/16/13 01:15 PM

That is funny!

Mike


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csrlice12
Postmaster
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Reged: 05/22/12

Loc: Denver, CO
Re: New Perspectives on Newtonian Collimation new [Re: Sarkikos]
      #5683624 - 02/16/13 02:22 PM



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