Dall-Kirkham telescope

 Diffraction limited flat-field of Dall-Kirkham telescopes compared to Newtonian telescope

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 (m2+1)

where :

F/D Newton = F/D ratio of the Newtionan telescope having the same diffraction limited flat-field

m = magnification factor of the DK secondary mirror

F/D DK primary= F/D ratio of the DK primary mirror

In 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.


Dall Kirkham 200 mm F/4-F15 :


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



Takahashi Mewlon 250 mm F/3-15 :

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.

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