- CDK20

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-Chrétien 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.