RE: [ATM] Large Secondary

["Peter" <peter@kitgear.com>]

Hi Bob,

I'm leaning toward giving this a try and your suggestion to mask the edge is
good. I'll leave a bit more meat and mask the edge. How much extra glass
should I leave?

I appreciate your input.


Peter

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of Bob May
Sent: Wednesday, June 29, 2005 3:16 PM
To: atm@atmlist.net
Subject: Re: [ATM] Large Secondary


Cutting an already finished optical surface will make the edges turn up a
bit.  This is caused by the grinding process making tiny cracks into the
surface and those cracks cause local stresses in the glass and that will
push the optical surface up for a tiny fraction of an inch in width of that
optical surface.  If you don't need the edge of the optical surface, then go
ahead and grind to the new shape and not worry about it.  It can be covered
up by the edge support of the secondary if you make the holder for the
secondary that way.  The Novak secondary holders used a tube with a rolled
over edge to hole the secondary so that the edge of the glass was covered
for about 1/8" or so.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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