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Most accurate way to center spot a mirror?

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34 replies to this topic

#1 epee

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Posted 03 July 2020 - 12:28 PM

Bought a Farpoint Cheshire but the included template didn't fit my mirror. My Orion XT12's mirror is 310mm or 12 3/16". I tried marking the provided template but must have messed up somehow

#2 markb

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Posted 03 July 2020 - 12:34 PM

Best way is to keep me as far away as possible.

 

I've never had much luck with using flat paper templates on curved mirrors or lenses.

 

Seriously, I thought the only hope for me, and perhaps for others that always seem to miss by a little bit no matter how carefully they measure, is a 3D printed slip over guide with a central hole.

 

I've been planning to make such a guide for a while but everything is on hold until I complete my move west, and for the first time read a post this week where someone had used the same method, but a big dob is still a challenge. It still is a viable option for spotting a secondary if desired.

 

Frankly, I'm surprised no one has yet offered such an item commercially.

 

A large dob is almost certainly in my future, so I will follow the thread.

 

All of the methods I seen so far require paper templates or a flat acetate template.

 

I always assumed that if I had the misfortune to be forced to center spot a large deep mirror, I would use two or three narrow strips of acetate with a center hole to admit a sharpie or something for a temporary mark, and make marks equidistant on the strip from the center hole on each side of the hole that would allow it to be lined up with the opposite edges of the primary. God willing the two or three center perforations would then coincide, confirming the measurements are accurate, and I could make a mark through the aligned holes.

 

Hopefully, long-term telescope makers have a far better method.


Edited by markb, 03 July 2020 - 12:47 PM.


#3 Cameron_C

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Posted 03 July 2020 - 12:53 PM

Maybe not accurate enough for you...

 

I just trace the outline f the mirror on a piece of paper. I sit it on the paper and draw around the outside.

The I fold the circle onto itself. And then I fold it onto itself again.

Where the two fold lines intersect is the centre of the circle.

 

I cut a tiny hole there for marking the mirror.


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#4 JohnBear

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Posted 03 July 2020 - 01:16 PM

My biggest primary is 8", so I pretty much use Cameron's method. For 6" or smaller mirrors, I found a second hand ($1) acetate embroidery circle template that works remarkably well.

 

With big parabolic mirrors, how can you be sure that the center if the parabola is really in the center of the mirror (for sub-mm precision)?   My guess is that you could lay the mirror flat, and truly level, and then put a small ball bearing/BB, etc., on the glass to gravitationally find the precise parabolic center. Then you could carefully make your reference marks around that.


Edited by JohnBear, 03 July 2020 - 01:17 PM.

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#5 Pinbout

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Posted 03 July 2020 - 01:36 PM

Bought a Farpoint Cheshire but the included template didn't fit my mirror. My Orion XT12's mirror is 310mm or 12 3/16". I tried marking the provided template but must have messed up somehow

I have a pdf you can take to staples and print on acetate 



#6 epee

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Posted 03 July 2020 - 01:39 PM

My guess is that you could lay the mirror flat, and truly level, and then put a small ball bearing/BB, etc., on the glass to gravitationally find the precise parabolic center. Then you could carefully make your reference marks around that.


Interesting idea!

#7 epee

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Posted 03 July 2020 - 01:39 PM

I have a pdf you can take to staples and print on acetate


Can I get a copy of that from you?
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#8 jtsenghas

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Posted 03 July 2020 - 02:03 PM

My biggest primary is 8", so I pretty much use Cameron's method. For 6" or smaller mirrors, I found a second hand ($1) acetate embroidery circle template that works remarkably well.

 

With big parabolic mirrors, how can you be sure that the center if the parabola is really in the center of the mirror (for sub-mm precision)?   My guess is that you could lay the mirror flat, and truly level, and then put a small ball bearing/BB, etc., on the glass to gravitationally find the precise parabolic center. Then you could carefully make your reference marks around that.

I wouldn't trust the curvature to be sufficient for accuracy to a few tenths of a millimeter. 

 

The problem with making a template, then dotting the center, then dotting equidistant from the center dot and then targeting the dots is the accumulated error. 

 

By the nature of the mirror making process, deviations between physical and optical centers are probably negligible, in my estimation. 

 

The method I like is a variant of Cameron's above. It has worked well for me for several large mirrors. 

 

I start with a piece of paper that is fairly robust, like cooking parchment paper, or coated butcher's paper. I once got a big square of this just for the asking at a deli. 

 

I then cut a PRECISE circle from it with a homemade trammel compass the size of the optical surface, smaller than the actual outside diameter by twice the bevel.  This compass can even be a push pin and an X-acto knife secured to a yardstick or long ruler. 

 

Such a template already will have a precise pinhole for the exact center. I then crease the paper into quarters as described and snip off a square with a single cut at precisely the outside radius of the centerspot. It's better to start small and increase it if needed.  The X-acto knife will cut squarer than scissors.  

 

Next I tape the template with a piece of painter's masking tape at each crease tightly to the mirror with the tape on the bevel and sides of the mirror. I make a fine ink mark across one piece of tape and its location on the glass as a reference for its location so I can replace it accurately for the next step. 

 

I peel three pieces of tape off the glass leaving attached only the one opposite the marked one and fold the template back. 

 

Next I use a trick from the instructions for the Catseye templates made for this purpose. I put a piece of the same tape across the outside of the square hole with the adhesive side facing the mirror side. I adhere the center marker to this tape centering it carefully.  It can be adjusted if necessary until exactly right. I then peel the adhesive film off the center marker if one is still attached. 

 

Holding the indexed tape I stretch the template tightly back into position, taping the other three pieces down starting with lining up the index marks. 

 

Now the center spot is suspended above the mirror by about its sagitta. I check carefully the placement of the template to the bevel and make final adjustments to those four pieces of tape if necessary. 

 

I push the spot down and apply pressure to it so that it is sticking to the glass more firmly than to the painter's masking tape (or any not too robust tape) on the template. 

 

Voila! Precision! 


Edited by jtsenghas, 03 July 2020 - 03:37 PM.

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#9 jtsenghas

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Posted 03 July 2020 - 02:15 PM

Frankly, I'm surprised no one has yet offered such an item commercially.

Catseye Collimation tools, Jim Fly's company, offers clear templates, perhaps printed on mylar, with many diameters marked on it. I'm not aware of large printed structures and really don't see the need. 

 

As an inherent tinkerer and DIYer, I prefer using his method with paper as described above.  The key is that the paper and tape get stretched the few thousandths of an inch to have the spot contact. You aren't just laying flat paper into a parabolic basin. You also aren't giving the spot a chance to contact prematurely if you stretch the template into place as I described. 


Edited by jtsenghas, 03 July 2020 - 03:33 PM.

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#10 jtsenghas

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Posted 03 July 2020 - 05:29 PM

It may be that for very large, fast mirrors my method would have to be used with the template not stretched quite as tight or with a very slightly more elastic material than I've used. The difference in arc length of a parabolic section and the diameter of most mirrors is trivial enough that I bet this usually works fine. 

 

You could test deflection of the template before adding the tape and center marker if needed and loosen the four pieces of tape a wee bit if needed. 



#11 BGRE

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Posted 03 July 2020 - 07:18 PM

Interesting idea!

Doesn't work well as there is no reliable reference (guaranteed parallel to the tangent to the at the mirror centre) for the level to rest on.
Also stiction degrades the accuracy of location of the centre with this method.
Try rolling a ball bearing on an inclined glass plane and you will find that there is a minimum slope required for the ball to start moving.

#12 B. Hebert

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Posted 03 July 2020 - 07:36 PM

I am not a mirror grinder, so I'm a bit ignorant, but:

 

How do you know that the physical center of the mirror is the center of the curvature?


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#13 BGRE

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Posted 03 July 2020 - 07:59 PM

In general you don't.
However the difference in location between the mirror pole (applies to paraboloids) and the centre of the aperture is often small.
A spherical surface has no unique axis or pole (point where the axis intersects the surface).

With a paraboloid there is a simple cheap and reliable way of aligning your mirror without a centre spot or any tools other than a camera (or equivalent).
Centre spots aren't always feasible such as for a Cassegrain or other telescope with a hole in the centre of the mirror.

#14 Pinbout

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Posted 04 July 2020 - 08:49 AM

Can I get a copy of that from you?

sent



#15 epee

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Posted 04 July 2020 - 12:11 PM

Got it!
Thanks

#16 Pinbout

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Posted 04 July 2020 - 01:37 PM

just make sure you cut the hole that's off center and use clear tape over the hole to grab and donut binder ring and place it by pressing down and burnishing the ring onto the mirror.

 

https://www.youtube....C3Kq-YETA&t=26s


Edited by Pinbout, 04 July 2020 - 01:38 PM.

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#17 123456

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Posted 16 July 2020 - 10:26 PM

What I do is basically do the mirror trace
On a soft piece of paper.
You know. Flip the mirror over onto the paper
And trace the edge
Do the double fold and do the tiny snip
Of the point. Then open it up. The tiny
Hole should look center.
Then cut the paper to look like a big X
With each of the 4 legs about 1 thick
The center should be about 1 round
Lay it on the mirror. It should lay right
Down to the center Then just match the
4 ends to the mirror edge You can use
Blue tape if needed then mark your center
If it’s a big mirror you can use 4 or 5
Legs. It always works for me
You may have to adjust your cuts as need
If you get kinks

#18 SteveV

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Posted 17 July 2020 - 01:45 AM

Starrett Center Finder

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#19 BGRE

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Posted 17 July 2020 - 02:09 AM

The faces of the Vee in that centre finder are intended to be tangential to the perimeter surface.
Thus it fails to work as the designer intended for cylinders with too great a diameter.
The solution is to use a larger Vee.

#20 Veni vini vici

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Posted 17 July 2020 - 03:47 AM

I used a circle with an attached non-waterproof fineliner pen and a flat piece of wood with a vee shape. I put the wood to the side of the mirror and draw a small part of a circle approximately at the middle of the mirror. I repeated that two more times approximately 120 degrees of each other. What you get is a small rounded triangle around the center of the mirror.
https://ibb.co/GTZkS61

Edited by Veni vini vici, 17 July 2020 - 03:51 AM.


#21 Pinbout

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Posted 17 July 2020 - 06:10 AM

I am not a mirror grinder, so I'm a bit ignorant, but:

 

How do you know that the physical center of the mirror is the center of the curvature?

If it’s not, the mirror sux.


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#22 Stardust Dave

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Posted 17 July 2020 - 06:06 PM

Any of my mirrors are round.

 

Rather than something like the starett center finder (similar idea), using a drafting ruler with 32nd's of inch on the Amercian side. It's corked backed so I lie it right on the mirror and press lightly several fingers to take out the belly to be precisely accurate .  Measure and mark center from 3 sides (lightly), I'm on center IMHO.

 

The John Hall primary in the big scope I'd measured this way. The provided center scribe mark was dead on to my measurement.

 

 


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#23 Jon Isaacs

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Posted 17 July 2020 - 08:23 PM

If it’s not, the mirror sux.

 

If the center marker and the optical center do not coincide, the images the mirror provides will suffer even it mirror is optically OK.

 

How does one determine the optical center of the mirror optically?

 

Jon



#24 kb58

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Posted 17 July 2020 - 09:05 PM

I was going to suggest putting a small bearing on the mirror and let it show you where the bottom is. Of course that may be different than the physical center, as mentioned. I also realized that it doesn't necessarily mean that the optical axis is 90° to the back of the mirror.


Edited by kb58, 17 July 2020 - 09:07 PM.


#25 BGRE

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Posted 17 July 2020 - 09:35 PM

If the center marker and the optical center do not coincide, the images the mirror provides will suffer even it mirror is optically OK.
 
How does one determine the optical center of the mirror optically?
 
Jon

You are actually trying to locate vertex of the paraboloid, the intersection of the axis of the paraboloid with the surface.

Coma is zero on the optical axis of a paraboloid when observing a star.
This can be used to accurately align a primary mirror as described in:https://iopscience.i...1086/663976/pdf


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