From: "Marco Miglionico" 
To: "Mark Holm" ,
        ,
        
Reply-To: "Marco Miglionico" 


Three points would probably be adequate for 25mm thickness (see plop plot).
However this may satisfy Raleigh's criterion, it does not satisfy the
requirements
I would put on a planetary scope having a diffraction limited performance.
If  I  had gone to the trouble of producing a mirror with a wavefront of
accuracy 1/8th to 1/10th wave which is considered real diffraction limited
performance, all that good work in parabolizing (theoretically) would be
wasted and at the mercy of your three point cell.

Lets say that the optical worker by, as you say luck, or skill does produce
an excellent figure, maybe one night at the eyepiece for a fraction of a
second the seeing steadies, then the 9 point cell will come into its own.

Personally I think you are right Mark. I've never been able to tell the difference
when I mounted my mirror (8.75in 25mm thick) on 3 or 9 point cells. But it
is not exacting or complex  to make the 9 point cell so what is there to
lose?

Lastly 25mm glass is 1.95 times stiffer than 20mm glass. There is no way I would put
a 20mm thick (8in dia) mirror on a 3 point cell.

----- Original Message -----
From: Mark Holm 
To: ; ; 
Sent: Thursday, May 08, 2003 3:39 AM Subject: Re: ATM First mirror
prjoject, how thin?


> Responding to Goran Hosinsky
> Marco Miglionico wrote:
>
> > Don't go thinner than 25mm.
> > Even mount this thickness of glass
> > on a 9 point cell for planetary viewing.
> >
>
>
> Goran,
>
> At 25mm thick and 200 mm diameter, a three point mounting with the support
> points located at 0.4 r  (40 mm radius), will be plenty good enough.  (I
am not
> being condescending.  It would be good enough for even the most demanding,
but
> still rational, ATM.  Even a professional would be satisfied with it,
unless
> perhaps he was going for diffraction limited work in the far ultraviolet!)
This
> is a result calculated by Plop.  See
> http://www.telerama.com/~mdholm/atm/cells/index.html
>
> You can get that 1/8 wave, but it isn't always easy.  Some folks are
either
> skillful or lucky and seem to get there with not too much trouble.  Others
(like
> me) go through a long procession of ugly figures before arriving.
>
> You can go thinner than 25 mm.  I haven't done it, but many others have.
20 mm
> should be doable in 200 mm dianeter, though that might be as thin as a
beginner
> should go.  If you can get 25 mm thick, it will be a little easier and you
won't
> have to be quite as careful.  Even at 25 mm, it is still quite possible to
grind
> in significant astigmatism if you don't take the standard precautions.
>
> More important than thickness is the uniformity and annealing of the
glass.  A
> well annealed, uniform 20 mm blank is vastly better than a poorly annealed
25 mm
> one.  A lot of the glass in this world is not well annealed by optician's
standards.
>
> You have some first rate observatories in the Canary Islands.  Do any of
them
> have a local optical shop?  If so, then they must get supplies.  Perhaps
they
> would be willing to add your order to one of theirs.  (Of course you pay
them.)
>   I am thinking that the shipping, and perhaps handling charges will be
lower if
> your order is part of a larger shipment.
>
> Mark Holm
> mdholm{at}telerama.com
>
>
>
>

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