Table of contents :


Newtonian telescope

Dall Kirkham 200 mm F/4- F/15, Mewlon 210 F/2.9-F11.9 and Mewlon 250 F/3-F15

Gregory 250 mm F/3.2-F/28.4

Cassegrain 200 mm F/4-F/15 and Cassegrain 350 F4.9-F29.2

Ritchey-Chretien 500 mm f/3-8

Celestron "classic" and Edge-HD versions overview

Celestron 8 classic and Edge-HD

Celestron 14

Celestron 14 with optimized Schmidt plate position

Celestron 8 with optimized Schmidt plate position

Celestron 14 classic and Edge HD

CDK 355 mm F3 - F11.8

Barlow lenses



500 mm f/3-8 Ritchey - Chretien telescope:


OSLO model:

NB: By comparison, the M1-M2 distance of the Astrosib RC 500 f/3-8 is 994 mm (instead of  1009.1 mm) while the backfocus (from the surface of M1)  is 335 mm (instead of  300 mm).

Sensitivity of Strelh ratio to backfocus:

The RC design is very sensitive to backfocus. In order to keep the Strelh ratio > 0.9, the backfocus should be within +/- 20 mm of its nominal value.


Radius of diffraction limited flat-field for nominal backfocus (Optical Path Difference should be < 0.07 to be diffraction-limited) :


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