Parabolic primary and elliptical secondary.
The field curvature is convex to the sky.
Main advantages :
- "easy" to make elliptical secondary mirror (compared to the hyperbolic secondary of classical Cassegrain)
- native long focal lenght : no need for additional refractive element in the optical path, which means top performance from UV to IR
Main drawbacks :
- optical tube longer than Dall-Kirkham or classical Cassegrain
OSLO model :
Diffraction limited flat field (550 nm) :
About +- 0.11°, equivalent to the diffraction limited field of a 250 mm F/6.7 Newtonian.
Sensitivity to backfocus :
This version offers a good flexibility of the back focus. An increase from 160 to 160 (resp. 260 mm) of the backfocus reduces the Strelh ratio from 1 to 0.99 (resp. 0.96).
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