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




This new telescope design looks very promising for HR imaging.


CDK 355 mm F3-F11.2 - Optical design : Vladimir Sacek


Central obstruction :


Radius of the diffraction limited flat field :

This design is diffraction limited over a large field and a large range of wavelenghts.

0.1° at 300 nm, 0.16° at 350 nm, and about 0.3° from 550 nm !!

Strehl ratio versus wavelenght :

This design is diffraction limited from over 300 nm to 1000 nm.

0.98 at 300 nm, then 1 from 350 to 1000 nm !!!

In comparison, the C14 or the C14 Edge-HD are diffraction limited on axis only between (about) 480 and 700 nm

Focus position versus wavelength :

The change of focus with wavelenght is negligeable.

0.01 mm difference over 350 nm to 1000 nm !!!

Field curvature :

- to be evaluated....

Sensitivity to backfocus (at 550 nm):

The range of backfocus with a Strehl > 0.95 on axis is relatively large (+/- 50 mm).

However if a large field is required (radius > 0.1°), the reasonnable backfocus range drop to about +/-20 mm.

To put things in perspective for high resolution work, a 2000 x 2000 pixels sensor with 5 microns pixel size would cover a maximum of 0.05° field in radius, ie 2.9 arcmin, (at 0.12 arcsec/pixel sampling). In these conditions, the backfocus range is greater than +/- 50 mm

Sensitivity to radial off-centering of the secondary mirror :

To be evaluated.....

Diffraction limited field with various Barlow lenses (and nominal spacing between M1 and M2):

The use of a Barlow lens dramatically reduces the radius of the diffraction limited flat field. However, the diffraction limited field is still way larger than the field covered by a "classic C14", and twice larger than the field covered by a CMOSIS 4000 sensor at 0.12 arcsec/pixel (ie. 2.9 arcmin at about X3 sampling) when used with the "long Barlow".




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