[ATM] Another flat-field anastigmatic aplant

["vladimir" <svla@isp.com>]

I touched this system before, but it didn't seem really workable.
But then, it may be. It is a 3-mirror system, consisting from
a concave ellipsoidal primary, convex spherical secondary
placed inside the primary focus, refocusing in the primary/secondary 
interspace and sending diverging beam through opening on the primary to the ellipsoidal tertiary 
that forms final image behind the primary. The final image can be made accessible  
with a diagonal flat.

The arrangement is quite similar to Willstrop's variant of Paul-Baker in mirror placement. 
The mirrors are somewhat easier to make, baffling is inherently easier due to configuration, 
but it also puts limit of somewhat over 1 degree diameter to the field w/acceptable edge vignetting
and up to ~50% c.obstruction.

An example is this 400mm aperture f/4.56 system:
R1=-2400mm
K1=-0.742
to secondary:-1017.5mm
R2=-550mm
K2=0
to tertiary:1600mm
R3=-713.5mm
K3=-0.373

Arrangement can have smaller or larger secondary/tertiary sets. The smaller they get, 
the more aspherising needed for the primary and tertiary (they remain ellipsoids). The system f# 
is nearly fixed in terms of primary's f#, being approximately 1.5 times "slower".

The flat-field Strehl .5 degree (16mm) off-axis is 0.995 for the above system.

Vlad
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