PlaneWave - 20" CDK Corrected Dall-Kirkham Carbon Fiber Truss Telescope

The new CDK optical design is the innovative solution for unsurpassed astroimaging quality at an affordable price. The purpose of the design is to provide a telescope that will excel at 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-Chretien design. The RMS spot sizes at the edge of a 35mm frame remain smaller than a single pixel on the most advanced CCD cameras available to amateurs today.

This no-compromise design is unique in making the optical alignment very forgiving and collimation very easy. This guarantees the user will be sure to get the best performance out of the telescope possible. 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), all the way out to a 42mm image circle. All this means, the stars will be pinpoints from the center of the field of view out to the corner of the field of view.

Features and Specifications

System

Aperture 20" (.51m)
Focal Length 3454 mm
Focal ratio f/6.8
Central Obstruction 39%
Back Focus 8.9" from focuser mounting surface, 5.81" from racked in focuser
Weight 140 lbs
OTA Length 47"
Upper Cage Carbon Fiber Truss
Lower Cage Carbon Fiber Truss with Carbon Fiber light shroud

Primary Mirror
Diameter 20.5"
Aperture 20"
Focal ratio f/3
Mounting Laser Collimated and Permanently Fixed
Material Precision Annealed Pyrex
Shape Prolate Ellipsoid
Coating Enhanced Aluminum - 96%

Secondary Mirror
Diameter 7.5"
Material Precision Annealed Pyrex
Shape Spherical
Coating Enhanced Aluminum - 96%

Lens Group
Diameter 90mm
Number of lenses 2
Coating Broadband AR Coatings (less than .5% from 400 to 700nm)
Standard Features
Dual 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 from 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
Dovetail The CDK20 comes with one of two dovetail designs. The first is the massive PlaneWave dovetail and the second is a dovetail compatible with Software Bisque's Versa Plate, VP dovetail. Both utilize the expansion joint listed above.
3.5" 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". (View picture) (View picture2)
Cooling Fans Three fans blow filtered air onto the back of the primary mirror to help it quickly equibrate to the ambient temperature. The fans are controlled by a switch on the optical tube or can be controlled by a computer if the optional EFA Kit is purchased.

The CDK telescope is a brand new optical design invented by Dave Rowe. The goal of the design is to make an affordable astrographic telescope with a large enough imaging plane to take advantage of the large format CCD cameras of today. Most telescope images degrade as you move off-axis from either coma, off-axis astigmatism, or field curvature. The CDK design suffers from none of these problems. The CDK is coma free, has no off-axis astigmatism, and has a flat field. The design is a simple and elegant solution to the problems posed above. The CDK consists of three components: an ellipsoidal primary mirror, a spherical secondary mirror and a lens group. All these components are optimized to work in concert in order to create superb pinpoint stars across the entire 42mm image plane.