PlaneWave CDK20/PW20-RC Optical Tube

PlaneWave CDK20/PW20-RC Optical Tube

PlaneWave CDK20/PW20-RC

The PlaneWave Instruments CDK20/PW20-RC is a 20 inch (0.51 m) f/6.8 Astrograph telescope in either Corrected Dall-Kirkham (CDK) or Ritchey-Chrétien (RC) optics. The telescope has a dual carbon-fiber truss design, with 3 cooling fans ejecting air from the back of the telescope. The CDK20/PW20-RC 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 inch Hedrick focuser.

The CDK 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-Chretien 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.

Fused Silica Glass Option: Fused silica is a synthetic amorphous silica glass of the highest purity and one of the most transparent glasses made. The optical and thermal properties of fused silica are superior to other types of glass due to its purity. Its transmission and homogeneity exceed those of crystalline quartz without the problems of temperature instability inherent in the crystalline form. Fused Silica has a coefficient of thermal expansion six times lower than Borosilicate (Pyrex) glass, which means that as fused silica cools down, it preserves its shape to a high degree of accuracy. This translates into consistent optical performance and unchanging focus over temperature changes. With its high melting temperature (~1,600 degrees Celsius), a very low coefficient of thermal expansion and resistance to thermal shock, Fused Silica is the material of choice for professional observatories as well as various scientific applications.

Cost: Adds $10,000 to the base price.


Ritchey-Chrétien Optics Option: In certain applications such as photometry and spectroscopy, refractive lenses could influence scientific results. The PW20-RC in its native format (no field flattening lenses) uses only a primary and secondary mirror with reflective coatings. This can be useful when making observations primarily on-axis and wide field imaging is not a concern.

Cost: Adds $13,500 to the base price.


Starting at $32,500
Corrected Dall-Kirkham / Ritchey-Chrétien Optics Comparison
PlaneWave CDK20 Brochure
PlaneWave Videos


Optical Design: Corrected Dall-Kirkham (CDK) or Ritchey-Chrétien (RC)

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 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 & Spot Size Plots; Spot and Diffraction Diagrams

Overall Dimensions: Overall Dimensions (PDF)


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)

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 or Fused Silica

Shape: Prolate Ellipsoid (CDK); Hyperbolic (RC)

Coating: Enhanced Aluminum - 96%

Secondary Mirror

Diameter: 7.5 inch (191 mm)

Material: Precision Annealed Borosilicate or Fused Silica

Shape: Spherical (CDK); Hyperbolic (RC)

Coating: Enhanced Aluminum - 96%

Lens Group (CDK)

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

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

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.

Included Accessories

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.

Planewave Thumb Drive: Containing all documentation and software 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)