**Table of contents :**

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

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

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

According to ref W1, a Dall-Kirkham telescope has a diffraction limited flat-field nearly than can be approximated to the one of a Newton having a F/D ratio equal to :

F/D

_{Newton}= 1.4 x m x F/D_{DK primary}/ sqrt (m^{2}+1)where :

F/D

_{Newton}= F/D ratio of the Newtionan telescope having the same diffraction limited flat-fieldm = magnification factor of the DK secondary mirror

F/D

_{DK primary}= F/D ratio of the DK primary mirrorIn the previous relation, the "m" secondary magnification factor have limited impact. The following figure is for m = 3:

We will see in the following OLSO simulations that the actual diffraction limited field departs a little bit from the previous approximated value.

OSLO model :From [1]

Radius of the diffraction limited flat field :About 0.09° at 550 nm (from OSLO simulation). This is equivalent to a 200 mm F/5.4 Newtonian.

Radius of the diffraction limited flat field with the "2X long Barlow" :Same as without Barlow lens from 400 to 700 nm (not simulated out of this range).

Field curvature :- 413 mm (from calculation),

- higher magnification of secondary miroir increases field curvature,

- higher focal ratio on primary miroir decreases field curvature.

Sensitivity to backfocus (wavelength = 550 nm) :This DK is pretty insentivive to reasonable variations of backfocus ; the Strehl ratio keeps greater than 0.99 for backfocus between -130 mm to + 170 mm from nominal back focus.

Sensitivity to radial off-centering of the secondary mirror :An off-centering of the secondary miroir can be pretty well corrected by adjusting the tilt of the secondary miroir as shown in the three following figures :

(a) Centered optics and nominal back-focus (550 nm) :

(b) With a 1 mm radial decentering of the secondary mirror (and refocussed on axis) :

(c) With a 1 mm radial decentering of the secondary mirror, and secondary mirror tilted by -0.13° to correct on axis image (and refocussed on axis) :The diffraction limited flat field in about 0.07° (in radius)

## Takahashi Mewlon 210 mm F/2.9-11.9 :

OSLO model :From Jocelyn Seérot and P. Cheyssac - Astrosurf Magazine n°48

__OSLO model :__

From [W3].

__Radius of the diffraction limited flat field :__

0.042° at 550 nm. This is equivalent to a 250 mm F/4.1 Newtonian.