[ATM] Aplantic folded Gregorian variety

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

Played with my favorite 2-mirror design, and come up
with folded 3-mirror+diagonal design that uses two
spherical (including primary) and one moderately ellipsoidal
mirror. The specs for an effective 300mm f/4.15 system are:

R1: -1800mm
K1: 0
primary-to-secondary: -825mm

R2: 360mm
K2:0
secondary-to-tertiary: 325mm

R3: -360mm
K3: -0.39

Small folding diagonal is placed just out of the focal zone of the secondary, directing light path towards the tertiary, placed out on the side. Tertiary forms accessible final focus on the opposite side. Strictly talking, it is not a Gregorian,
being a 3-mirror system, but might be considered a cousin of some kind.

The system is corrected for spherical and coma, with remaining strong astigmatism. Diffraction limited field
over best image surface (276mm curvature) is only ~4mm,
or 0.15 degrees (more than double vs. comparable Newtonian). However, with an ordinary eyepiece, having
opposite strong astigmatism/field curvature, a visual
field quality would be substantially better. 

Effective obstruction comes from the folding diagonal
in the light converging from the primary, and can be bellow
30% with this particular system. Haven't examined what
would be smallest obstruction achievable with the design.

Advantages vs. Newtonian are better field quality, more
compact size and easier to make mirrors. Disadvantages 
are somewhat larger c.obstruction (likely), more complicated mechanics and considerably greater miscollimation sensitivity.

Vlad
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