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How to Collimate your Newtonian

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#151 Elric82

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Posted 07 September 2012 - 05:13 AM

Thanks Jim, I hope you're right

#152 Blaise

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Posted 11 September 2012 - 05:05 PM

Does anybody use blue loctite on the locking screws? I'm having problems with mine backing out after long trips in the car.

#153 Peter Trinidad

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Posted 14 November 2012 - 06:18 PM

Hi Vic:

I am having the hardest time trying to collimate a 3 inch, f/6 old red tube Edmund telescope which is from around the 1980's (about 18 inches long). There are several of them under the reflector section on cloudy nights.
The mirror is not adjustable and is set in one position with screws. The only mirror you can adjust by moving it left or right or by bending the metal stalk which is the secondary mirror. At the present time I am using a small, plastic eyepiece with a pinhole in the middle of it but at best adjusting the secondary the stars look like "seagulls" and the moon looks out of focus at best. Someone told me that if you cannot adjust the primary mirror that it would be very difficult to collimate.

What are your thoughts?

Peter

#154 sixela

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Posted 15 November 2012 - 10:31 AM

That is correct: if you can move only one mirror you can get rid of coma in the centre of the image or focal plane tilt but to eliminate both you need to be lucky...

I'd just determine where the focal plane is and use a Cheshire or collimation cap with its pupil close to that focal plane; centre the collimation cap pupil in the centre spot or make the centre spot concentric with the Cheshire ring. And live with the focal plane tilt...

If you don't have a centre spot either then it's even more difficult to do something sensible, really.

#155 Vic Menard

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Posted 15 November 2012 - 10:42 AM

Hi Peter,
Without a closer look at the mechanicals, I have to agree with Sixela's suggestions.

The only other possibility I could think of was to look at the way the primary mirror is mounted in the OTA (I couldn't find a good picture online). If it's cemented in place, your options are much more invasive. But if it's mounted in place with a few screws it may be possible to widen the screw holes enough to provide a small amount of tilt adjustment to the mirror/cell assembly. That's the way my refractor lens is mounted and it has plenty of tilt adjustment...

#156 redsun

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Posted 01 December 2012 - 06:58 PM

hey Vic i just replaced the 8" mirror on my orion reflector along with new spiders i have collamated it like i always do the when i insert a 25mm and try to focus an object 1/4 mile away it will not come in to focus. im not new to reflectors owend them since i was 18 now 55 lol i need held

#157 CatseyeMan

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Posted 01 December 2012 - 08:17 PM

hey Vic i just replaced the 8" mirror on my orion reflector along with new spiders i have collamated it like i always do the when i insert a 25mm and try to focus an object 1/4 mile away it will not come in to focus. im not new to reflectors owend them since i was 18 now 55 lol i need held


Jumping in, I suspect your new mirror has a shorter or longer focal length than the original which has put your 25mm out of focus range. If the image gets sharper racking the focuser in, loosen all 3 Primary collimation screws about 5 turns, recollimate the Primary and try again to focus. If the image gets sharper racking the focuser out, tighten all 3 collimation screws about 5 turns, recollimate the primary and try again to focus.

#158 Jason D

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Posted 01 December 2012 - 08:34 PM

Tell us more about your mirror replacement. Did you take the focal length into account as Jim mentioned?

#159 hoa101

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Posted 14 December 2012 - 06:38 PM

I had a quick question about the "carefully decollimated" procedure that may be answered somewhere already... just cannot find it.

Anyway, after we've aligned the center spot with the sight tube, and the axial alignment with the procedure, won't the diagonal mirror always end up with the center spot no longer perfectly centered in the crosshairs of the sight tube? Anytime you adjust the tilt of the diagonal, it seems like it will end up a touch off. Or at least it does whenever I try to collimate in this manner on my dob.

Hopefully somebody can set me straight. Thanks.

#160 Starman1

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Posted 14 December 2012 - 07:00 PM

The alignment of crosshairs with center spot on primary makes sure the focuser axis is aligned with the optical axis.
When you use the cheshire, next, to center the primary's center marker in the dark center of the bright annulus in the cheshire (i.e. if you have a combination tool you ignore the crosshairs for this part), you align the optical axis of the primary with the optical axis from the focuser.

The CDP procedure is used if you have an autocollimator, and the tiny tweaks necessary to correct the secondary won't really throw the crosshairs of the sight tube out enough to misalign the crosshairs and center marker.
You only decollimate the primary when doing this procedure so you can see the two images from the secondary and stack them perfectly. THEN you re-collimate the primary to stack all 4 images.

If you don't have an autocollimator, CDP has no meaning. Then it's simply:
--align secondary until crosshairs and primary mark line up.
--align primary until center mark is in center of dark area in cheshire.

#161 hoa101

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Posted 14 December 2012 - 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?

#162 Starman1

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Posted 14 December 2012 - 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.

#163 Allan...

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Posted 01 May 2013 - 01:06 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

#164 beatlejuice

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Posted 01 May 2013 - 01:31 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

#165 Jason D

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Posted 01 May 2013 - 01:56 AM

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

#166 beatlejuice

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Posted 01 May 2013 - 01:24 PM

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

#167 michaeldurban

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Posted 17 October 2013 - 03:18 PM

you lost me at mechanical focusser axis...

#168 Vic Menard

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Posted 17 October 2013 - 03:55 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.

#169 obin robinson

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Posted 27 May 2014 - 05:31 AM

For anyone wanting extremely accurate collimation I posted a thread on the ATM forum concerning the construction of a collimation webcam. These are extremely easy and cheap to make. It gets rid of all of the guesswork and inaccuracies which other methods may have. I figured that since this is an important collimation thread for Newtonians that this information would be of value here.

http://www.cloudynig...6547389/page...

obin :)






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