From: "Martti Koskimo" 
To: 
Reply-To: "Martti Koskimo" 



Plaster base for prism and small lens making has been used more than
hundred years with success.
Your 'lens' is too large and too thin. There is a difference in thermal
expansion and it is impossible to keep temperatures in perfect control .
Also other uneven forces in Plaster/glass joint worsens the situation.
It works for multiple glass pieces but not for single THIN big mirror(
obviously you don't want to brake you blank to smaller pieces).

I made a 5 inch thick plaster table( 800 mm diameter for 800 mirror) and
ceramic tiles sinked in the plaster. The mirrors backside  was ground
against the ceramic covered plaster table for perfect fit.
Although there is perfect fit there are always inevitable some particles
between table and mirror and two hard surfaces can in this case have no
even contact.
So a foam rubber was inserted between table and mirror. This is quite good
but not perfect. The foam rubber
is very even but the hardness is not perfect evenly distributed. So there
is still need to rotate the mirror on the table. All this labor but not
very much profit.

Next I poured Pitch on the 800 mm tool (10 mm thick) grooved the pitch  to
squares and the mirror on it.
Heavy paper was inserted between pitch and mirror to be able to lift the
mirror for testing.
This did not cure the astigmatism. The Pitch was probably too hard and I am
too impatient to wait hours and
days for perfect contact.This method probably works if properly don .In my
case again much labor but no success. Softer pitch and more
patience............
I have learned  that Pitch is NOT always flowing evenly. Extended Cold
pressing is no guarantee for perfect fit. Warm pressing is always needed to
guarantee perfect contact.

Now I use always flotation systems  if the mirror is 300 mm or bigger. (54
point for 800 mm).
Astigmatism is best handled this way ( In my case at least) This is not
perfect system.
In basic polishing when hard lateral forces must be used the mirror has a
tendency to rotate from the table and this tendency is asymmetric. In
figuring this support is PERFECT. Don't use mere screws and nuts under the
triangles. That is not sensitive enough. I did learn this ( and so many
other things) the hard way. Finally I made the bearing joints  from  cone
shaped polished steel( like bullet) tenons  and this is put inside  cone
(bigger angle) shaped Teflon. There is no problem in fixing the Teflon.
Using screw threaded Teflon bar it is trivial to  screw it to any material
( even in Teflon) This is like a dream bearing. So good that I converted
all my mirror flotation systems to the same.


Martti Koskimo




 ----- Original Message -----
From: "Vladimir Galogaza" 
To: "ATM shore" 
Sent: Friday, December 27, 2002 2:27 PM Subject: Re: ATM Astigmatism on 20
inch thin mirror


>
> >.  The thing that has to be
> >considered is that the glass is very thin so you are going to have to
make
> >sure that the glass is properly supported or you will just put a the
> >astigmatism back in in a different orientation.
>
> How to properly support the blank?
>
> Would it be proper to make a very thick disk from dental stone
> (extra hard variety) or cement, say 2 or even 4 inches thick
> than after cured put on it  a fresh layer of dental stone sufficiently
thick
> to accommodate
> unevenness of the blanks back and put blank on it. Dental stone will
> presumably
> take the exact form of the mirrors back ( this is what dental stone is
used
> for by dentists)
>  and when cured will  simulate thick mirrors stiffness ( to the point).
>
> Now I ask experts to tell us what stiffness ( if any) will have this
> composite
> expressed in  equivalent thickness of some hypothetical pure glass blank.
>
> Of course this composite will serve only for  grinding, polishing,
figuring.
> Later thin mirror should be separated and placed in one of those
> multihundred point cells.
>
> Regards
> Vladimir.
>
>
>
>
>
>
>

--- BBBS/NT v4.00 MP