[ATM] A new flat-field anastigmatic aplant?

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

Never heard of this kind of arrangement, but seems to be
workable. A near parabolic primary can be turned into a
flat-field anastigmatic aplant with a pair of spherical mirrors:
a small convex secondary placed shortly after the prime 
focus, and concave tertiary facing it. The specs of an
effective f/3.71 300mm aperture system are:

R1: -2425mm
K1= -0.95
primary-to-secondary: -1322mm

R2: -350mm
K2=0
secondary-to-tertiary: 277mm

R3: -410mm
K3=0

Flat-field spot size is still less than 1 micron 10mm 
(0.5 degrees) off-axis, and the only limitation field-wise comes
from vignetting. 

It doesn't work quite as well with a paraboloid: coma or astigmatism
can be corrected only alternatively. Still, field performance is significantly
better. For instance, tangential coma at 10mm off-axis in an effective 
300mm f/4 system is only about 1/4 wave (~1.6 times the Airy disc diameter,
or less than 10microns), with astigmatism/field curvature corrected.

Although final image could be made accessible for visual observing,
this is primarily an imaging system, due to relatively large c.o. (~50%).
Both, secondary and tertiary need to be inside a separate closed
structure, to ensure good baffling.

It seems to be scaling fairly well with the F#.

Vlad
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