A four-mirror anastigmat aplanat telescope can be constructed using a
Gregorian-form Mersenne afocal telescope as a feeder for a concentric
spherical Schwarzschild objective. A flat image field can be obtained
by adjusting the ratio of the radii of the mirrors. With the tertiary
at the image of the aperture stop (the primary) the primary’s
conic can be varied, using the
tertiary’s conic to cancel the primary’s spherical
aberration without affecting the system’s coma or astigmatism.
For a spherical primary, the tertiary is an oblate ellipsoid
(). The minimum obscuration ratio is 44.7%
(20% by area).
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