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PlaneWave CDK 20" Telescope Optical Tube Assembly

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SKU 1323220Q

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  • A 20"/0.508m f/6.8 visual and astrographic telescope
  • Uses PlaneWave's Corrected Dall-Kirkham (CDK) optical design
  • 70mm diameter imaging circle
  • No field curvature, off-axis coma, or astigmatism.
  • Carbon fibre truss-tube for low thermal expansion, rigidity and weight saving.
  • Two optical glass options available

PlaneWave's CDK20 is a 20 inch (0.508 m) aperture f/6.8 telescope that is very well suited for astro-imaging as well as for visual use.

The CDK20 telescope utilises the Corrected Dall-Kirkham (CDK) optical design, developed by Dave Rowe, that provides excellent imaging possibilities with large format CCD cameras, while remaining superb for visual use. The CDK design far exceeds the off-axis performance of most commercial telescope designs including the Ritchey-Chrétien design making the CDK an innovative solution for unsurpassed astro-imaging quality at an affordable price. 

The end result, at the image plane of the CDK20, will give you pinpoint stars from the center to the corner of its 70mm field of view without any field curvature, off-axis coma, or astigmatism. The no-compromise optical design is unique in making the optical alignment forgiving and collimation very easy which guarantees the user the best possible performance from the telescope. 

The CDK20 features a dual carbon-fibre truss design, with 3 cooling fans to help cooling of the primary mirror. The instrument weighs 63.5kg (140lbs) and comes as standard with a back plate retaining ring ready to accept the focuser of your choice. 

Telescope Features

The CDK20 telescope is an open truss tube design for a rigid, but lightweight structure. The truss tube utilises a carbon fibre frame which minimises thermal expansion that can cause focus shift with changes in temperature. The carbon fibre truss tube design also promotes quick thermal cooling.

CDK Carbon Fibre Truss Poles

The CDK's carbon fibre truss poles 

To create the telescope's baffles, Planewave uses digital 3D printing technology. Their 3D printers add successive layers of material to construct a baffle system with precision. These internal stray light baffles minimise vignetting and maximise image contrast.

CDK 3D Printed Baffle

3D Printed Primary Baffle

To mount the CDK20 onto a mount, PlaneWave supplies the CDK with a dovetail bar. The CDK features a dovetail expansion joint that allows for the difference in thermal expansion between carbon fibre and aluminium. This expansion joint allows the aluminium dovetail expand and contract without stressing the carbon fibre lower truss.

CDK Expansion Joint

Dovetail expansion joint

For dew prevention, the CDK20 is internally wired with polyimide film heater pads and temperature sensor, ready to be controlled with the optional Delta-T controller.

CDK Internal Heater Pads for Primary

Internal heater pads for dew prevention

To help the primary mirror to cool, three cooling fans eject air from the back of the telescope and helps the telescope to reach thermal equilibrium quickly. The fans can be controlled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased.

CDK Rear Cooling Fans

The CDK 20" cooling fans 
Optical Features

The Ritchey Chrétien (RC) optical design has long been "the standard" in high end imaging telescopes due to its use in many professional observatories. The RC is successful in eliminating many of the problems that plague many other optical designs, namely off-axis coma, however the RC design does nothing to eliminate the damaging effects of off-axis astigmatism and field curvature. The RC design is also very difficult, and therefore expensive, to manufacture and optically align. 

CDK Optical Design

Optical Design of the PlaneWave Corrected Dall-Kirkham (CDK)

The CDK design tackles the "off-axis coma problem" by integrating a pair of correcting lenses into a two mirror design. The beauty is that this design also corrects for astigmatism and field curvature. Because the lenses are relatively close to the focal plane (unlike the Schmidt corrector plate found in various Schmidt Cassegrain designs), and because these lenses work together as a doublet, there is no chromatic aberration. The CDK offers a wide aberration-free, flat field of view that allows the user to take full advantage of the very large imaging chip cameras in the market place today.

CDK Spot Diagram

In the image above, are two simulations showing the CDK’s stunning optical performance (CDK20).The left column is a diffraction simulation and the second in the right column is a spot diagram. In both of the simulations the small squares are 9x9 microns, about the size of a typical CCD pixel. In the diffraction simulation the star images on axis and off-axis are nearly identical. In the spot diagram 21mm off-axis the spot size is an incredible 6 microns RMS diameter. For 26mm off-axis, a 52mm image circle, the RMS spot size is 11 microns. This means the stars across the entire focal plane are going to be pinpoints as small as atmospheric seeing will allow. The spot diagram was calculated at 720, 585, and 430nm (many companies show spot diagrams in only one wavelength, however to evaluate chromatic performance multiple wavelengths are required). Both of the simulations take into consideration a flat field, which is a more accurate representation of how the optics would perform on a flat CCD camera chip.

Having an aberration free telescope design means nothing if the optics cannot be aligned properly. Many Ritchey owners never get to take full advantage of their instrument’s performance because the Ritchey is very difficult to collimate. Aligning the hyperbolic secondary mirror’s optical axis to the optical axis of the primary mirror is critical in the Ritchey design, and the tolerances are unforgiving. The secondary mirror of the CDK design is spherical. It has no optical axis and so the centering tolerance of the CDK secondary mirror is comparatively huge. With the help of some very simple tools, the CDK user will be able to set the secondary spacing, collimate the optics and begin enjoying the full performance potential the instrument has to offer within a few minutes.

There are 2 options available for the type of optical glass used in the CDK20 telescope: Borosilicate and fused Silica (see below) plus a fused Silica optic-only option for fork mounts

Below are a few images (ARP316 top; Crescent nebula bottom) taken with a CDK20 to show how well they perform. 

ARP316 Taken with CDK20

NGC6888 (Crescent Nebula) taken with CDK20



Aperture 20 inch (508 mm)
Focal Length 3454 mm (135.98 inch)
Focal ratio F/6.8
Central Obstruction 39% of the Primary Mirror Diameter
Back Focus from Racked in Focuser 5.8 inch (147 mm)
Weight 140 lbs (63.5 kg)
OTA Length 47 inch (1,194 mm)
Upper Cage Carbon Fiber Truss
Lower Cage Carbon Fiber Truss with Carbon Fiber Light Shroud



Diameter 7.5 inch (191mm)
Material Fuses Silica
Shape Spherical
Coating Enhanced Aluminum - 96%



Optical Diameter 20 inch (508 mm)
Outer Diameter 20.5 inch (521 mm)
Shape Prolate Ellipsoid
Material Fused SIlica
Coating Enhanced Aluminum - 96%



Diameter 90 mm (3.54 inch)
Number of lenses 2
Coating Broadband AR Coatings (less than .5% reflected from 400 to 700nm)



Crated Shipping Weight 291 lbs   (132.0 kg)
Crate Width 33 inch   (838 mm)
Crate Height 40 inch   (1,016 mm)
Crate Length 65 inch   (1,651 mm)


Ronchi Spacer (200354) This spacer sits in place of a focuser and is used for setting the primary to secondary spacing. It has an 1 .25”  inner diameter and may be used with 1.25” oculars for collimation.
Ronchi Ocular This ocular is a Ronchi screen used for setting the primary to secondary spacing.
Primary Mirror Cover  The Primary Mirror Cover protects the primary mirror.
Printed Instructions  For Collimation and Spacing
12VDC Power Supply (18778)  Provides power for the fans (Not included for European Orders)
Wrench Set (5812A35 )  English Hex Wrenches (European Orders Only)


Drawings & Diagrams


Camera Adapters Diagrams



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