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

["Francis J. O'Reilly" <foreilly@verizon.net>]

Rien,

I believe that the center of the glass will necessarily coincide with the 
center of the mirror unless rather unusual grinding techniques are applied.

FJO
----- Original Message ----- 
From: "marinus van der Lugt" <lugt@xs4all.nl>
To: "Guy Brandenburg" <gfbrandenburg@yahoo.com>
Cc: "ATM List" <atm@atmlist.net>
Sent: Monday, September 03, 2007 1:08 PM
Subject: Re: [ATM] marking the center of a circle (was: Re: 
UltimateCollimation)


> Hi Guy,
>
> That will give you the center of a circle, but will it give you the
> center of the mirror?
> The center of the mirror must not necessarily coincide with the
> center of the glass.
>
> Rien.
>
>
> On Sep 3, 2007, at 18:46, Guy Brandenburg wrote:
>
>> 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
>>>> _______________________________________________
>>>> ATM mailing list http://www.atmlist.net/
>>>>
>>>>
>>>
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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
>>
>> =============================
>> "There are enough stars in the universe that if everybody
>> on Earth were charged with naming his or her share, we'd
>> each get to name a trillion-and-a-half of them."
>> - wrote Anthony Doerr in Orion magazine, reprinted in
>> Utne Reader, Sept-Oct 2007, page 91
>> =============================
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>
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