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PlaneWave Instruments 20 inch CDK Optical Tube Assembly

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Product Description

The PlaneWave Instruments CDK20 is a 20 inch (0.51 m) f/6.8 Corrected Dall-Kirkham Astrograph telescope. The CDK20 has a dual carbon-fiber truss design, with 3 cooling fans ejecting air from the back of the telescope. The CDK20 covers a 52 mm field of view without any field curvature, off-axis coma, or astigmatism. The instrument weight is 140 lbs (64 kg) and comes standard with the large capacity 3.5" Hedrick focuser.

The CDK (Corrected Dall-Kirkham) Optical Design is an innovative solution for unsurpassed astroimaging quality at an affordable price. The CDK telescope design provides excellent imaging 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.

This no-compromise design is unique in making the optical alignment forgiving and collimation very easy. This guarantees the user the best possible performance from the telescope. The end result at the image plane of the CDK design is no off-axis coma, no off-axis astigmatism, perfectly flat field (no off-axis defocus). The CDK design will give you pinpoint stars from the center to the corner of the field of view.

More about the CDK optical design.




20 inch   (508 mm)

Focal Length

3454 mm   (135.98 inch)

Focal Ratio


Central Obstruction

39% of the Primary Mirror Diameter

Back Focus from Mounting Surface

8.8 inch   (224 mm)

Back Focus from Racked in Focuser

5.8 inch   (147 mm)


140 lbs   (64 kg)

OTA Length

47 inch   (1,194 mm)

Upper Cage

Carbon Fiber Truss

Lower Cage

Carbon Fiber Truss with Carbon Fiber Light Shroud

Optical Performance

Vignetting and Spot Size Plots

Overall Dimensions

Overall Dimensions (PDF)



20.5 inch   (521 mm)


20 inch   (508 mm)

Focal Ratio



Laser Collimated and Permanently Fixed


Precision Annealed Pyrex


Prolate Ellipsoid


Enhanced Aluminum - 96%



7.5 inch   (191 mm)


Precision Annealed Borosilicate




Enhanced Aluminum - 96%



90 mm   (3.54 inch)

Number of lenses



Broadband AR Coatings (less than .5% reflected from 400 to 700nm)


Carbon Fiber Truss Design

Minimizes thermal expansion which causes focus shift with changes in temperature

Carbon Fiber Lower Light Shroud

Protects the primary mirror from damage and stray light

Dovetail expansion joint

Allows for the difference in thermal expansion between carbon fiber and aluminum. The expansion joint allows the aluminum dovetail expand and contract without stressing the carbon fiber lower truss

3.5 inch Hedrick Focuser

Heavy duty no-slip focuser. The focus tube runs on 5 bearings and is driven by a leadscrew so there is no chance of slipping. It accepts an optional dial indicator and PlaneWave’s EFA Kit to control many electronic accessories. The draw tube travel is 1.3 inch.

Cooling Fans

Three fans blow out of the optical tube pulling air though the telescope and by the primary mirror. This helps the telescope to reach thermal equilibrium quickly. The fans are controlled by a switch on the optical tube or can be conrolled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased.


Crated Shipping Weight

291 lbs   (132 kg)

Crate Width

32 inch   (813 mm)

Crate Height

36 inch   (914 mm)

Crate Length

59 inch   (1,499 mm)


3.5" to 2" Adapter (200349)

Adapts the 3.5" inner diameter of the focuser to a standard 2" inner diameter. This adapter is necessary when setting the primary to secondary spacing. The 200399 Visual 3.5" to 2" adapter is an optional accessory that is designed to space a 2" diagonal for visual use.

Rochi Spacer (200354)

This spacer is used for setting the primary to secondary spacing. It has an 1-1/4 inner diameter and may be used with 1-1/4 oculars for collimation.

Rochi 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)

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