Re: [ATM] Figuring support cell

["Marco" <marco.miglionico@btinternet.com>]

Hi Mike,

thanks for that detailed answer. It's starting to become clearer now.
You are right I think it was a hill Stathis got. Interesting that it
also happened on your 30in. It really is pushing the limits of what you
can do on a wooden table!

> Apply polishing pressure in addition and the mirror will flex downward
> between supports (and the cell may flex if not designed properly).
> The mirror will be high over the support points, and those high spots
> will be polished away, causing print-through.

Yes I agree, but what does the carpet do when you are pushing on it?
It 'pushes back' at the localised area of presure, just like a cell may.
Only with a cell, you know exactly where the weight is distributed 'on 
average'.

I think if you say that print through will happen at the cell support point,
then logically you are implying that the carpet supports the mirror more 
evenly
than a cell would - which may be correct. I simply do not know which is
why i'm asking.

> There are also the lateral polishing forces to consider - apply some
> pressure to a lap and you will pull the mirror side to side, and the
> cell would have to be up to the task of preventing the mirror from
> moving around

Yes this is the biggest problem I can envisage. But then again cleats are 
used
in normal instances.

Marco.







----- Original Message ----- 
From: "Mike Lockwood" <melockwo@uiuc.edu>
To: "ATM LIST" <atm@atmlist.net>
Sent: Tuesday, October 17, 2006 9:54 PM
Subject: Re: [ATM] Figuring support cell


> Hi,
>
> Marco wrote:
>> When final polishing to sphere and then parabolizing, makers of
>> large thin mirrors often use a system of ~50% tool diameter, Tool
>> On Top or (TOT) strokes. As the strokes are not performed with a
>> full size lap Mirror on Top or (MOT), the thin mirror has to be
>> supported by something, usually a thick worktop with a piece of
>> carpet on it.
>
> When the mirror is flat on its back, the threads in the carpet act as
> thousands of individual support points, supporting the mirror quite
> evenly over a wide range of polishing pressures.
>
> Yes, the mirror can bend if lots of pressure is used, and this can
> cause problems.  Proper technique will cause these "problems" to be
> rotationally symmetric, and they can be removed during figuring.
>
> A good optician knows how to tell when he/she is using too much
> pressure for a given mirror thickness.
>
>> Now if one was to support a telescope mirror IN the mirror cell
>> with just a piece of carpet on a flat surface - then one or two
>> people might argue that this would impart a certain amount of
>> unacceptable deformation to the wavefront of light. Recommended
>> accuracy is 1/4 wave.
>
> If the mirror was flat on its back (telescope pointed straight up),
> the carpet would work well.  However, if you tilt the telescope, the
> mirror shifts position on the carpet (because the carpet acts like a
> spring), and the collimation would be lost.  I suspect the support
> would still work fine, though, and the mirror's shape would be OK.
>
>> Recently for example, Stathis Kafalis had to modify the table under
>> his 32in mirror due to a persistent central depression being caused
>> by his very thick table sagging very slightly. I've heard many
>> similar stories for thin mirrors of 12in and greater.
>
> Hmmm.... a sagging table should cause a central hill, not a hole.
> Also, the fact that large mirrors are appreciably thinner in the
> center makes them more flexible there.  Both this or a sagging table
> would makes the glass tend to flex downward under the weight of
> polishing, and then when pressure is removed the glass pops back up,
> resulting in a high center (oblate sphere).  This happenned on our 30"
> f/3.8 and on other thin mirrors I have made.
>
>> That said, I am suggesting the support of the mirror during these
>> final stages (TOT) could be done with a support cell similar to or
>> with even more support points than that which will be used to
>> support the mirror in the telescope.
>
> The support points of the cell are designed to provide a small
> distortion ONLY under the weight of the mirror itself.  Apply
> polishing pressure in addition and the mirror will flex downward
> between supports (and the cell may flex if not designed properly).
> The mirror will be high over the support points, and those high spots
> will be polished away, causing print-through.
>
> There are also the lateral polishing forces to consider - apply some
> pressure to a lap and you will pull the mirror side to side, and the
> cell would have to be up to the task of preventing the mirror from
> moving around.  My mirror cells typically use low-friction materials
> for the contact points (teflon) which allows the mirror to move freely
> in its cell without binding.  This would not work well for polishing.
>
> Mike Lockwood
> http://bi-staff.beckman.uiuc.edu/~melockwo/index.html
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/ 

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