I recently read in another thread about the best design for astrophotography, that concerning off-axis correction, probably the best native(!, so including [uncorrected] RCs [I assume corrected RC are again better than CDKs]) optical design of them all is the CDK (which I consider a native system, because a CDK without it's integrally designed corrector is absolutely worthless, so inferior and not even equal to a native Dall-Kirkham).
I wonder if it is possible to design a Gregorian telescope as fast as f/5? Whether this is possible with the same level of correction as a CDK and if this would require two or even three corrective lenses?
Just have found this interesting website http://ganymede.nmsu...tes/node16.html. It says that there are two classes of two-mirror telescopes: classical Cassegrain and classical Gregorian. The classical Cass has a parabolic primary mirror and a hyperbolic secondary. The classical Gregorian has a parabolic primary and ellipsoidal secondary. The aplanatic (spherical aberration and coma corrected) versions of the classical Cassegrain have a hyperbolic primary and hyperbolic secondary, better known as Ritchey-Chrétien. The aplanatic Gregorians have an ellipsoidal primary and ellipsoidal secondary.
The website states concerning optical quality:
[...] The image quality is clearly better for the aplanatic designs than for the classical designs, as expected because coma dominates off-axis in the classical design. In the aplanatic design, the Gregorian is slightly better. However, when considerations other than just optical quality are considered, the Cassegrain usually is favored : [...]
Is the following ranking (increasing towards perfection) correct concerning optical off-axis quality: CC, RC, CDK, corrected RC? Where in this ranking would the classic Gregorian, aplanatic Gregorian, and two/three lenses corrected aplanatic Gregorian be ranked? Mostly interested in the questions before, but for the sake of completion: what about those Harmer-Wynne and Riccardi-Honders products offered by several manufacturers in the suggested ranking?
Edited by Lucullus, 26 June 2021 - 03:16 PM.