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How to mark mirror center point?

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#1 Gschnettler


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Posted 24 October 2020 - 08:49 AM

I have a 6” dobsonian. I’d like to do laser collimation but that seems to require a dot or circle at the center of the primary mirror and mine doesn’t have that.

How would you recommend that I (a) determine the precise center of the mirror and (b) what should I use to mark it?

#2 Kyphoron


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Posted 24 October 2020 - 08:57 AM

I normally take the mirror out, trace around the mirror on a piece of paper. Then cut out the trace. Fold the circle in half and then in half again. Cut the little corner off. Unfold the paper, lay it over your mirror and where the hole is mark your center point with either a sharpie or a donut sticker.

Edited by Kyphoron, 24 October 2020 - 09:10 AM.

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#3 eyeoftexas


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Posted 24 October 2020 - 09:08 AM

You can use the little donut reinforcement stickers for binder paper.  Or, you can buy inexpensive templates and stickers from Catseye or Farpoint.

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#4 Asbytec


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Posted 24 October 2020 - 09:14 AM

The perforation in the primary center mark needs to be at least (0.02 f^3)/4. So at f/8 that's going to be 2.5mm from dead geometric center. The reason is, with perfect primary axial collimation using the central donut, any error with placement of the donut translates into primary axial error. The geometric center will be off by that much. You want to keep that minimal. With some care, you can get it a little closer. Especially if your scope is faster than f/8. So cut a very small corner off the folded paper template for better accuracy.

Edited by Asbytec, 24 October 2020 - 09:22 AM.

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#5 Junoscope



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Posted 26 October 2020 - 06:12 PM

The traced circle method can work well, but there is a bit of wiggle room in placing circle back on the mirror and placing the Sharpie dot.  Check your work by measuring the location at right angles with a ruler or tape measure, taking care with the mirror surface.  It’s quick and you may end up redoing your center dot. Placing a white paper ring with a tweezers after the center is determined helps visibility of the center.  

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#6 Starman1


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Posted 27 October 2020 - 02:14 PM

When placing the paper on the mirror, through which you make the Sharpie dot, I scotch tape the paper to the edge of the mirror to guarantee it stays centered and doesn't move.

After the dot is placed, the paper and tape are removed and I apply a paper reinforcement ring with tweezers to guarantee I place the dot on the mirror precisely at the center of the ring.

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