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


Gregory telescopes :


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



Gregory 250 mm F/3.1-F 28.4 :


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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