20 inch CDK Optical Tube Assembly

20 inch CDK Optical Tube Assembly

Part   200101

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.
The CDK Optical Design

FEATURES AND SPECIFICATIONS

SYSTEM

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 Mounting Surface	8.8 inch   (224 mm)
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
Optical Performance	Vignetting and Spot Size Plots Spot and Diffraction Diagrams
Overall Dimensions	Overall Dimensions (PDF)

PRIMARY MIRROR

Diameter	20.5 inch   (521 mm)
Aperture	20 inch   (508 mm)
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 inch   (191 mm)
Material	Precision Annealed Borosilicate
Shape	Spherical
Coating	Enhanced Aluminum - 96%

LENS GROUP

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

STANDARD FEATURES

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	The massive PlaneWave dovetail supports the optical tube to provide rigidity. A matching dovetail clamp is available to install on various mounts. Drawing (PDF)
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. Image 1 Image 2 Drawing of Flange Drawing of Draw Tube
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.

SHIPPING

Crated Shipping Weight	291 lbs   (132.0 kg)
Crate Width	32 inch   (813 mm)
Crate Height	36 inch   (914 mm)
Crate Length	59 inch   (1,499 mm)

INCLUDED ACCESSORIES

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.
Ronchi 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.
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)