From: Mark Holm 
To: ea8yu{at}arrl.net, atm{at}shore.net, marco.pm{at}btopenworld.com
Reply-To: Mark Holm 


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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