PlaneWave Instruments 20 inch CDK Optical Tube Assembly
Manufacturer : PlaneWave Instruments
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 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.
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 (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)
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 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.
SHIPPING
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)
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.
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)
