[ATM] 8" Diagonal- More info

[Polaraligned <polaraligned@optonline.net>]

Thanks to all  who responded to my inquiry for thickness of
8" diagonal.  Everyone who responded wants more info to
 be able to help me.  So here it is:

I ordered a 28" f/3.7 Steve Kennedy mirror.  They are 
fantastic by every account.  This is to be
used with a custom made 4" Wynne corrector designed
by Phillip Keller and being manufactured by LOMO in 
Russia.  With this combination I will be able to illuminate
a STL-11000 with only a few percent vignetting at the edges.
The corrector design produces some nice off axis spots and
has enough backfocus to allow the new Research Series AO to
be used between the corrector and camera.  
In order for all this to work, I need an 8" minor axis diagonal.
The major axis presents itself at about 11.4".  I understrand that
this is a large and difficult to make flat.  I am not capable
of making this myself.  I thought about maybe Newport Glass, but
I am receptive to anyone capable of making this flat with high
quality.  Any suggestions would be welcome.

It was suggested that an ellipse be cut out of a sheet without
"beveling" the ellipse back as is common in smaller diagonals.
This would make it easier to get a better figure at the edge where
the glass gets real thin.  The  negative I see with this is that 
it increases the CO.  For an 8" diagonal that only blocks an 8"
circle, the area is about 50 sq in and linear obstruction is 28.6%.
For an ellipse with no beveling, the area of obstruction is 62 sq in
and linear obstruction is 31.3%.  Quite a big increase.  The diagonal
also presents itself as an oblong obstruction instead ofcircular.
I thought about
having the ellipse beveled so that instead of getting super thin at the
bottom edge, the mirror would be say 1/2" thick at minimum.
Any thoughts? 

For support I would guess that Silicone is the number one choice
along with safety clips.   I have no idea how to space the support
points out without elliptical PLOP.   I was thinking of erring on the 
large side and maybe going with a 2" thick diagonal.  Of course,
thinner would be preferred for weight and cool down reasons.

As for the substrate, would there be any distinct advantage with 
using anything other than Pyrex?  

Holding 1/10 wave or better is the priority and I know that the 
edges are going to fade from this value.

Thank You,
Scott







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