[ATM] 25" f/5 folded newtonian

[polaraligned at optonline.net (polaraligned)]

I did a search for folded newtonians and came up with some design software
here: http://www.oldham-optical.co.uk/Design.htm
Looks like a 9" flat and a 4" diagonal make a very nice setup.  The
eyepiece is over 4' lower than a standard newtonian with this 25" f/5
mirror, and there are no abberations introduced.  The only negative I see is
that the obstruction ratio is 35.9%.  I remember reading somewhere that in
larger scopes like this the obstruction ratio does not matter as much.  I
don't know if this is true.  At that, the eyepiece being 51" lower seems to
make the folded newtonian of this design (see software), a real home run.
Why do I not see more of this design out there?  Am I missing something? How
much does a 9" optical flat cost?
I am strongly considering this design and would apprectate any input.


Thank You,
Scott