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Equipment Discussions >> Reflectors

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hoa101
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Reged: 02/04/12

Loc: Northern Virginia
Re: How to Collimate your Newtonian new [Re: Starman1]
      #5571408 - 12/14/12 07:38 PM

I have the full catseye set actually, including the new two-holed auto-collimator. You're right that the adjustments from the procedure do not throw the crosshairs off much. Usually they still hit the center spot somewhat. Usually on the edge or something, not centered. I've heard the secondary alignment does not need to be very accurate, but what is accurate enough?

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

Loc: Los Angeles
Re: How to Collimate your Newtonian new [Re: hoa101]
      #5571477 - 12/14/12 08:19 PM

The tiny tweaks necessary to the secondary in the lateral pupil of the XLK are typically so small the resolution of the crosshairs wouldn't allow you to see the crosshairs drifting away from the center of the center marker.

So what you have are two tools that don't agree.

First, are you tightening the focuser's setscrew on the tools? If not, the different weights of the tools could result in different registrations. You don't need to really haul down on the setscrews, but they should be snugged against the tools so you can't move the tools in the focuser.

Second, have you made sure that when the teletube was expanded that it was not off angle? Measure the distance from the shoulder of the tool to the end of the tube in several places and make sure it's the same to your ability to read the length. If it isn't straight, loosen the tiny screws and adjust it until it is.

Third, check the accuracy of the XLK autocollimator. Loosen the setscrew, rotate it in the focuser 90 degrees and retighten. It should give the same reading all the way around. If it doesn't, it's possible it's not sitting flat in the focuser or that the mirror on the inside isn't dead on. If you suspect that, return the XLK to Jim to have him check it.

Fourth, I presume you are lining up the crosshairs near your eye (the ones hard to focus on) with the distant center mark? The distant, smaller, reflected image of the crosshairs won't line up until the primary is collimated.

Fifth, have you ever checked to make certain your primary mirror's center marker is centered? It could be off a bit.

Sixth, after aligning the secondary and primary and then inserting the AC, how far off is it? Are the 4 images of the center marker a jumble in the center, or are they way off? If they're way off, then you may not be reading the sight tube or cheshire accurately enough.
On my own scope, when I take care with the sight tube and cheshire, the 4 images are almost stacked.

Seventh, when you do the CDP procedure, how close to already stacked are the two images remaining in the center? I see mine nearly stacked and only a tiny tweak of the secondary is necessary to line them up.
You could ignore CDP entirely, tweak the secondary to line up the images in the XLK's lateral pupil, then realign the primary with the cheshire, followed by the secondary/lateral pupil of the XLK, followed by the cheshire, and repeated until neither tool shows any change is necessary (the tweaks get smaller and smaller with each iteration). The central pupil of the XLK will then show just one image of the center marker on a jet black background, and if you reinsert the sight tube, the crosshairs should line up perfectly on the center marker.


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Allan...
sage


Reged: 10/24/12

Loc: Penticton B.C. Canada
Re: How to Collimate your Newtonian new [Re: Starman1]
      #5833579 - 05/01/13 02:06 AM

I know this question belongs in the beginners section but the thread was already started here, so here is my question (as odd as it might seem). Is it best to collimate (I use a laser) INside my residence before moving the scope (8" dob) outside, or wait til I'm outside; just in case I might bump it on the way out (its only a 50 ft trek and so far haven't hit anything). Im hoping the answer is INside as thats where I have been doing it so far. thanks, Allan

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beatlejuice
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Reged: 04/05/11

Loc: Hamilton, ON,Canada
Re: How to Collimate your Newtonian new [Re: Allan...]
      #5833596 - 05/01/13 02:31 AM

Unfortunately I am pretty sure that most of us collimate when the scope is at its final resting place for observing. But that doesn't mean that your 8 inch won't hold its inside collimation pretty well as you carefully take it outside. Its just better in the long run to check it again when you get there.

Eric


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Jason D
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Reged: 10/21/06

Loc: California
Re: How to Collimate your Newtonian new [Re: Allan...]
      #5833608 - 05/01/13 02:56 AM

Quote:

so here is my question (as odd as it might seem). Is it best to collimate (I use a laser) INside my residence before moving the scope (8" dob) outside,



This is a common question in forums -- not as "odd" as you might think.
The answer is complete the initial collimation inside then take your scope outside and wait until it cools down before checking collimation one more time. It might need little collimation touch up.
I suggest pointing the OTA at 45 degrees angle when collimating.
Jason


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beatlejuice
Pooh-Bah
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Reged: 04/05/11

Loc: Hamilton, ON,Canada
Re: How to Collimate your Newtonian new [Re: Jason D]
      #5834526 - 05/01/13 02:24 PM

Quote:

The answer is complete the initial collimation inside then take your scope outside and wait until it cools down before checking collimation one more time. It might need little collimation touch up.




I never thought about that Jason. Assuming the scope is not moved in the interim what happens in the 1-2 hours of cooldown to change the collimation?

Eric


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michaeldurban
member


Reged: 03/06/12

Loc: Durban, South Africa
Re: How to Collimate your Newtonian new [Re: travvy]
      #6143265 - 10/17/13 04:18 PM

you lost me at mechanical focusser axis...

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Vic Menard
Post Laureate
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Reged: 07/21/04

Loc: Bradenton, FL
Re: How to Collimate your Newtonian new [Re: michaeldurban]
      #6143316 - 10/17/13 04:55 PM

The "mechanical" focuser axis is what you set up mechanically (with rulers, squares, etc.) usually relative to the OTA (and sometimes relative to the mounting itself). The term is often described as "squaring" or "orthogonal" correction. The optical focuser axis is set up with a laser, sight tube, or autocollimator relative to the center of the primary mirror which is the origin of the primary mirror (or optical) axis.

Ideally, the two are the same (the mechanical and optical focuser axes). In practice, there is often some variance.


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