[ATM] marking the center of a circle (was: Re: Ultimate Collimation)

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Here's one fairly easy method:
Put the mirror onto a large piece of paper and trace around it with a pencil or pen. Remove mirror. Cut out circle. Then fold the circle in half, and crease (this gives you a diameter). Unfold it. Fold the circle in half in a different manner, and crease, giving you a different diameter. Repeat a few times if you feel like it. Where all of these diameters meet will be the center.
Cut a small hole where the center is. With the paper unfolded, lay it carefully right on top of the mirror, centered. Then mark the center, or scrape it, or whatever it is you want to do.
This should only take a couple of minutes, and is much easier than the procedure of constructing perpendicular bisectors of chords.

"Lawrence D. Lopez" <lopez@mv.mv.com> wrote: It need be no more accurate than the placement
of a center mark would be.

Still I always find this a problem.

Drawing a circle with a compass the same size
as the mirror might be used to do this.

Dropping perpendiculars on the centerline chords
I've always found to be a chore.

marinus van der Lugt wrote:
> Hi John,
>
>  > 1) remove reflective coating in center of mirror, about 1/8 inch
>
> You have to be quite sure about the center though.
> Is there a good way to find it?
>
> Rien.
>
>
> On Sep 2, 2007, at 15:26, johncdeitz@comcast.net wrote:
>
>   
>> Hello Gang: Try this for the ultimate in Newtonian Collimation-
>> 1) remove reflective coating in center of mirror, about 1/8 inch
>> 2) if mirror cell is solid provide hole through cell at center
>> 3) shine a light at the hole from behind the cell (I use an LED  
>> with variable brightness so as to preserve vision- this fits in  
>> snug hole at center of the back of mirror)
>> 4) using an autocollimator eyepiece, with the optics roughly  
>> aligned, you will typically find two spots of light from the LED  
>> (or flashlight etc.). Tweak the mirror to bring these two into one  
>> and you will see an additional reflection that is quick to change  
>> position- this is very sensitive method! Bring all three head-on  
>> and your view is flooded with light as with conventional  
>> autocollimator. You will find enough play in the autocollimator  
>> that his spot will wink on and off as you play with the  
>> autocollimator in the focuser.
>> This method offers the following advantages:
>> 1) you can follow the whole action continuously as the cell is  
>> tweaked, unlike with the conventional use of autocollimator
>> 2) can be performed in suboptimal seeing with accurate results
>> 3) no problem with off-set.
>> 4) can be performed without ruing night vision so you will not  
>> hesitate to repeat.
>> 5) can even be done by one person!
>> John C. Deitz
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>>
>>     
>
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Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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