A spherical secondary makes the Dall-Kirkham easy to make and to collimate
Horace Dall designed the system in 1928 and Allan Kirkham popularized it. An ellipsoidal primary and a spherical secondary are easier to figure and test than the two hyperbolas of a Ritchey-Chrétien, and a spherical secondary has no axis to align, so collimation is forgiving. On axis the image is perfect; away from the center, coma grows quickly, which is why the classic version suits the planets and double stars.
PlaneWave's Corrected Dall-Kirkham adds a two-lens corrector near focus that removes the coma, astigmatism and field curvature. The result is a flat field as wide as a full-frame sensor, and the design is more forgiving to collimate than an RC.
Takahashi makes the classic Mewlon; PlaneWave made the corrected version a standard
| Model | Maker | Design | Aperture | Focal length | Price |
|---|---|---|---|---|---|
| Mewlon 210 | Takahashi (Japan) | Classic Dall-Kirkham | 210 mm | 2,415 mm (f/11.5) | $3,170, special order |
| Mewlon 250CRS | Takahashi (Japan) | Dall-Kirkham with built-in corrector | 250 mm | 2,500 mm (f/10) | special order |
| CDK14 | PlaneWave (USA) | Corrected Dall-Kirkham | 356 mm | 2,563 mm (f/7.2) | $18,500 |
| CDK17, CDK20, CDK24 | PlaneWave (USA) | Corrected Dall-Kirkham | 432 mm to 610 mm | f/6.5 to f/6.8 | observatory |
| iDK | AG Optical (USA) | Imaging Dall-Kirkham | 254 mm and up | f/6.7 | dealer quote |
| RiDK | Officina Stellare (Italy) | Corrected Dall-Kirkham | 300 mm and up | varies | special order |
Prices are US list prices checked in November 2026. Special-order instruments are quoted by the dealer.
PlaneWave CDKs are common in the remote observatories in Chile, Spain and New Mexico where amateurs rent telescope time.
At 2,415 mm a Mewlon frames globular clusters, planetary nebulae and small galaxies
On an APS-C camera with the Sony IMX571 sensor (23.5 x 15.7 mm, 3.76-micron pixels), a Mewlon 210 at 2,415 mm gives a field of 0.56 x 0.37 degrees at 0.32 arcseconds per pixel. 80 of the 110 Messier objects fit in the frame, and 69 of them fill at least a quarter of its short side, among them M13 (Hercules Cluster), M43 (De Mairan's Nebula), M55 (Summer Rose Star) and M15 (Great Pegasus Cluster).
That image scale is finer than the sky allows, so long-focus Dall-Kirkhams are imaged with large pixels, binned 2x2, or reduced. The classic Mewlon's off-axis coma also needs Takahashi's reducer-flattener for anything beyond a small sensor.
Most backyard skies blur stars to 2 to 3 arcseconds. Image scales below about 0.7 arcseconds per pixel record no extra detail, so cameras on long focal lengths are usually binned 2x2.
The classic Mewlon is a planetary telescope; the CDK is not a visual buy
A Mewlon 210 is a favorite planetary telescope: shorter and cheaper than a refractor of similar resolving power, and easy to keep collimated. A CDK's large obstruction and price make no sense for the eyepiece: an observer spending that much should buy a large Dobsonian and a good refractor.