Table of contents :
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 with optimized Schmidt plate position
Celestron 8 with optimized Schmidt plate position
Celestron 14 classic and Edge HD
CDK 355 mm F3 - F11.8
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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