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PSA - Collimating your scope using the center spot on mirror

Beginner Collimation Reflector
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#1 Bener

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Posted 27 January 2022 - 12:28 PM

Hello folks, a PSA regarding Collimating your scope

 

As we all are well aware, Newtonian scopes require collimating to be able to provide the best views a given scope is capable of.  A fundamental aspect of collimation is utilizing a mirror’s center point as the visual cue to indicate when the mirrors are properly aligned (a bit of simplification, but appropriate for our purpose here).

Many of us probably assume that the mirror’s center spot is accurately placed during assembly and collimate our scopes based on that assumption.  But…..

 

I recently removed the mirror of my OneSky and checked the position of the factory applied center spot on the mirror.

 

It was off by about 1/4” (6.35mm) on a 5" mirror.  So, if you want the best views possible, don’t assume. Verify that your center spot is accurately located.


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#2 MellonLake

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Posted 27 January 2022 - 04:30 PM

You can check this pretty easy with a collimation cap and cell phone camera.  Take a picture through the collimation cap hole.  Then annotate the image in Word.  Put a circle on top of the primary mirror circumference and then lines for diameters at 90 and 180 on the circle (crosshairs).  The crosshair lines will cross where the centre of the mirror marker should be.  If they do cross at or near the centre of the mirror marker you are good.  If they are off, you may want to move the marker but that would also depend on your mirror size and focal ratio (large slow mirrors have wide tolerances for marker placement).

 

Rob


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#3 CHASLX200

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Posted 27 January 2022 - 07:11 PM

I think most are off anyways.


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#4 Vic Menard

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Posted 27 January 2022 - 07:39 PM

You can check this pretty easy with a collimation cap and cell phone camera.  Take a picture through the collimation cap hole.  Then annotate the image in Word.  Put a circle on top of the primary mirror circumference and then lines for diameters at 90 and 180 on the circle (crosshairs).  The crosshair lines will cross where the centre of the mirror marker should be.  If they do cross at or near the centre of the mirror marker you are good.  If they are off, you may want to move the marker but that would also depend on your mirror size and focal ratio (large slow mirrors have wide tolerances for marker placement).

waytogo.gif

Whenever I annotate someone's alignment picture, the first circles I draw are the primary mirror center marker and the actual edge of the primary mirror to verify the centering of the primary mirror center marker.

 

At f/5, the allowable error (for high magnification performance) for the primary mirror tilt adjustment is ~0.7mm. If the primary mirror center mark is 6mm off center and the primary mirror is collimated "perfectly" to that reference, the induced primary mirror tilt error will be 1/2 the centering error (3mm). It's pretty easy to place a new center marker within 1mm, and for a small aperture mirror, 0.5mm accuracy isn't that much harder.


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#5 Bener

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Posted 27 January 2022 - 08:19 PM

waytogo.gif

Whenever I annotate someone's alignment picture, the first circles I draw are the primary mirror center marker and the actual edge of the primary mirror to verify the centering of the primary mirror center marker.

 

At f/5, the allowable error (for high magnification performance) for the primary mirror tilt adjustment is ~0.7mm. If the primary mirror center mark is 6mm off center and the primary mirror is collimated "perfectly" to that reference, the induced primary mirror tilt error will be 1/2 the centering error (3mm). It's pretty easy to place a new center marker within 1mm, and for a small aperture mirror, 0.5mm accuracy isn't that much harder.

Of course Mr. Menard points out the key issue here.  The reason I took out my mirror was to place a center mark that came with my recent purchase of better quality collimating tools.  Before learning that my center spot was off, my perfectly collimated OneSky scope (at least I thought it was) gave me less than stellar sharpness with my wider FOV lenses and I couldn’t figure out why.  Once I got things straightened out with the real center used, I felt I had almost gotten a new scope.


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#6 Asbytec

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Posted 28 January 2022 - 01:30 AM

I took a picture of my mirror (removed from the scope) from several feet away, then did as Rob suggested. Except I put a (red) square around the the mirror with each side just grazing the edge of the mirror. Then dropped in some diagonal lines from corner to corner. Did the same with the primary center mark as shown. With some care, you can get good measurements. You can even count pixels in some PC image programs. I think I was using GIMP. 

 

Center Spot HD Annotated (Zoom).jpg



#7 Stonius

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Posted 28 January 2022 - 01:40 AM

Is it at all possible that the manufacturer's spot is set to denote the optical axis of the mirror rater than the rotational axis?

 

Is there any guarantee that a finished mirror's optical axis lies at the exact centre of the blank?

 

Markus



#8 Asbytec

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Posted 28 January 2022 - 04:43 AM

Is it at all possible that the manufacturer's spot is set to denote the optical axis of the mirror rater than the rotational axis?

 

Is there any guarantee that a finished mirror's optical axis lies at the exact centre of the blank?

 

Markus

I seriously doubt mass production places the marker with such care. Premium makers probably do by virtue of their quality fabrication, so we trust the rotational axis is likely very well centered on the mirror, anyway. No guarantee with mass production. We can only assume this to be the case. I don't know of anyone who may have used a laser or cross hair to pinpoint their axis of rotation after careful collimation on a star, but it might be doable. 



#9 Stonius

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Posted 28 January 2022 - 06:10 AM

I wonder if you could get a feel for it by placing the mirror on a turntable such that the rotational axis is at the centre of the dot. A laser reflected from the exact centre of the dot should show no wobble in reflection. Then try it again where the mirror is centered on the turntable, ignoring the spot. That would tell you if the rotational center of the mirror is the same as the optical.

Does that make sense as a way of testing?

#10 Asbytec

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Posted 28 January 2022 - 06:21 AM

I wonder if you could get a feel for it by placing the mirror on a turntable such that the rotational axis is at the centre of the dot. A laser reflected from the exact centre of the dot should show no wobble in reflection. Then try it again where the mirror is centered on the turntable, ignoring the spot. That would tell you if the rotational center of the mirror is the same as the optical.

Does that make sense as a way of testing?

I recall someone mentioning doing something similar, but I don't remember what the challenges were or what the outcome was. You could try it, then check it by measuring it with a template. You can trace the mirror on a piece of paper, fold it twice, then snip a small piece of the corner. That'll put a hole in the center of the template. But then, just use the template to find the geometric center and assume it's the optical center. I am not familiar with the challenges of using a laser to find the optical center of rotation on a turntable. It sounds complicated. 


Edited by Asbytec, 28 January 2022 - 06:24 AM.


#11 briansalomon1

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Posted 28 January 2022 - 08:08 AM

I wonder if you could get a feel for it by placing the mirror on a turntable such that the rotational axis is at the centre of the dot. A laser reflected from the exact centre of the dot should show no wobble in reflection. Then try it again where the mirror is centered on the turntable, ignoring the spot. That would tell you if the rotational center of the mirror is the same as the optical.

Does that make sense as a way of testing?

If you put the mirror on a rotation table and aligned a laser with the outside edge you could move the mirror until the laser just grazed the edge of the mirror as it rotated. Then it would be centered on the table.

 

Then you could align the laser with the center marker and any movement would show how far away it was from the physical center of the mirror.

 

But any wobble in the table would translate to error and with the lasers we have it's hard to get better than ~0.5mm precision. It would be much simpler and about as accurate to use the paper template Asbytec mentioned.

 

The cellphone method Rob mentioned is interesting and doesn't risk touching the mirror.


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#12 Bener

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Posted 29 January 2022 - 11:55 AM

Perhaps a less geeky or fun way to determine the center is to use a template designed for this purpose—like the one that comes with some of the collimating tools—or measuring and simple geometry to determine the center of a circle.

 

Granted, not as much fun as using a turntable or lasers, but maybe easier for some.

 


Edited by Bener, 29 January 2022 - 11:58 AM.


#13 MellonLake

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Posted 29 January 2022 - 02:33 PM

Perhaps a less geeky or fun way to determine the center is to use a template designed for this purpose—like the one that comes with some of the collimating tools—or measuring and simple geometry to determine the center of a circle.

 

Granted, not as much fun as using a turntable or lasers, but maybe easier for some.

I found the templates not sufficiently accurate.  The method I describe with a camera is simple and works better.

 

Rob


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#14 SteveG

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Posted 29 January 2022 - 09:38 PM

Perhaps a less geeky or fun way to determine the center is to use a template designed for this purpose—like the one that comes with some of the collimating tools—or measuring and simple geometry to determine the center of a circle.

 

Granted, not as much fun as using a turntable or lasers, but maybe easier for some.

The templates work poorly IMO. I got mine close, but photos through my Cheshire show it to be very slightly off. I have an f5 that I use with a Paracorr, so it needs to be nearly perfect.


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#15 Mike Lockwood

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Posted 31 January 2022 - 01:29 PM

I have dealt with center spotting for quite some time.

 

There is no way to determine if the optical center of a mirror is off unless you do a lot of measurements with a very good interferometric test system, such as the one that I now use.  Even with that, it is labor intensive and local mirror distortions will make it very difficult.  Trust me, I know.  This approach is futile.

 

If you use a laser and look at how it deflects as the mirror rotates, then wedge in the blank (one side thicker than the other) or a turntable that isn't perfectly perpendicular to the shaft that it is mounted on can throw off the laser beam.  There is no easy way to tease out where the errors are.  This approach is also futile.

 

The best way to center spot a mirror is to put it on a rotating table and use a $20 dial indicator or indicator to simply mechanically center it.  If the blank is not round, then this will be a problem without a good solution, and you'll have to do the best you can.  This is why a properly machined blank is important.

 

After it is centered, ideally to better than 0.005", which takes only a few minutes, you can then mark or scribe a circle that is exactly centered on the mirror.  This is one of the last steps that I do when I finish a mirror - I scribe small, precisely centered circles into the glass so that they are permanent.  Then the client's choice of center spot can be easily and accurately applied with the circles as a precise reference.


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#16 hoa101

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Posted 31 January 2022 - 02:56 PM

My OneSky mirror center spot was also off quite a bit geometrically. I forget how much, but it was shockingly large from what I recall.

 

I had to re-spot after getting my largest mirror coated and did the best I could with the tools available. I try not to worry too much about it, perfection is the enemy of good enough, etc.



#17 Bener

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Posted 31 January 2022 - 03:10 PM

Thanks for the input everyone.  There's so much to learn in this hobby :-)

 

I did use a template but tonight I will verify the location of the center with some measurements. 

 

@hoa101, my OneSky's mirror was off by about 1/4” (6.35mm) on a 5" mirror!  Which is why I started this thread -- I was shocked.



#18 Spikey131

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Posted 31 January 2022 - 03:36 PM

I have dealt with center spotting for quite some time.

 

There is no way to determine if the optical center of a mirror is off unless you do a lot of measurements with a very good interferometric test system, such as the one that I now use.  Even with that, it is labor intensive and local mirror distortions will make it very difficult.  Trust me, I know.  This approach is futile.

 

If you use a laser and look at how it deflects as the mirror rotates, then wedge in the blank (one side thicker than the other) or a turntable that isn't perfectly perpendicular to the shaft that it is mounted on can throw off the laser beam.  There is no easy way to tease out where the errors are.  This approach is also futile.

 

The best way to center spot a mirror is to put it on a rotating table and use a $20 dial indicator or indicator to simply mechanically center it.  If the blank is not round, then this will be a problem without a good solution, and you'll have to do the best you can.  This is why a properly machined blank is important.

 

After it is centered, ideally to better than 0.005", which takes only a few minutes, you can then mark or scribe a circle that is exactly centered on the mirror.  This is one of the last steps that I do when I finish a mirror - I scribe small, precisely centered circles into the glass so that they are permanent.  Then the client's choice of center spot can be easily and accurately applied with the circles as a precise reference.

Time to dust off my old Yamaha turntable……grin.gif



#19 Mr.Furley

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Posted 02 February 2022 - 10:48 PM

I believe that the center spot on my 10" GSO mirror is etched.. tried to get it off for a bit but gave up & stuck the triangle on top of it.

Hope it's centered but who knows.
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#20 Bener

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Posted 02 February 2022 - 11:20 PM

My OneSky had a center spot that was a glued on ring that came off after soaking for a while in soapy water. My other scope has an etched center circle. I simply applied the new center triangle mark (there’s a few different shapes) in the correct location, which was just a few millimeters from the etched location.  The reflective surface of the replacement spot makes it much easier to collimate, especially in the dark.



#21 CrazyPanda

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Posted 03 February 2022 - 12:10 AM

I think most are off anyways.

Agreed. Now and again people will post pictures of their collimation on Reddit. Where possible, I'll load their image into Photoshop and draw virtual crosshairs and circles to highlight misalignment.

 

What I find most interesting is if I draw an outline around the reflection of the primary mirror, and then draw crosshairs that snap to the center of that circle, it becomes immediately clear when the center spot is not actually centered, and most are off by at least 1/8th of an inch.

 

This is true more often than not. I would wager all factory applied center spots are simply not accurately applied.



#22 Poynting

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Posted 03 February 2022 - 11:39 AM

Two data points:

 

My cheap GSO 6" F/6 has an etched circle and using a Catseye Template I have verified it is nearly perfectly placed relative to the mirror's edge.

 

On the other hand, my GSO 10" F/4 Carbon Truss Conical mirror has an etched circle that is off by quite a bit. The mirror has a thread on the back that screws into the back plate on the scope, and when it rotates the error is obvious as the center spot wobbles around. I ended up putting a hotspot over it in the correct place.

 

In this instance you wonder what is more likely: GSO took the time and effort in centering the spot to the actual optical center of the mirror, OR the person who center spotted the mirror was sloppy on this one.

 

It warps my mind that they wouldn't take care when doing an etched spot that is permanent.


Edited by Poynting, 03 February 2022 - 12:14 PM.


#23 CrazyPanda

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Posted 03 February 2022 - 01:38 PM

I have dealt with center spotting for quite some time.

 

There is no way to determine if the optical center of a mirror is off unless you do a lot of measurements with a very good interferometric test system, such as the one that I now use.  Even with that, it is labor intensive and local mirror distortions will make it very difficult.  Trust me, I know.  This approach is futile.

 

If you use a laser and look at how it deflects as the mirror rotates, then wedge in the blank (one side thicker than the other) or a turntable that isn't perfectly perpendicular to the shaft that it is mounted on can throw off the laser beam.  There is no easy way to tease out where the errors are.  This approach is also futile.

 

The best way to center spot a mirror is to put it on a rotating table and use a $20 dial indicator or indicator to simply mechanically center it.  If the blank is not round, then this will be a problem without a good solution, and you'll have to do the best you can.  This is why a properly machined blank is important.

 

After it is centered, ideally to better than 0.005", which takes only a few minutes, you can then mark or scribe a circle that is exactly centered on the mirror.  This is one of the last steps that I do when I finish a mirror - I scribe small, precisely centered circles into the glass so that they are permanent.  Then the client's choice of center spot can be easily and accurately applied with the circles as a precise reference.

In case anyone is curious, here's what Mike does with his mirrors:

https://www.cloudyni...8148_303176.jpg

The inner edge of the outer circle perfectly aligns with the hole of the standard Cat's Eye Hotspot.

 

Makes dead-accurate application of the center mark a breeze:

https://www.cloudyni...148_1621273.jpg
 


Edited by CrazyPanda, 03 February 2022 - 01:39 PM.



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