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PlaneWave CDK12.5 OTA

12.5" (315mm) x 2541mm (f/8)


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 52mm diameter image circle yield are 12 microns, about the size of a pixel of most CCD cameras.


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 52mm 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.


Price correct as of August 2013. Please contact us for a quote and to discuss other options available for Planewave's CDK telescopes. Some options such as the DeltaT temperature monitoring and dew control system must be included at the time of purchase.

Carbon Fiber Tube 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.



The CDK Optical Design












The CDK [Corrected Dall-Kirkham] telescope is based on a new optical design developed 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 52mm image plane.



Optical Performance


Shown are two simulations showing the CDK's stunning performance. The first is a diffraction simulation and the second is a spot diagram. In both simulations the small squares are 9x9 microns, about the size of a CCD pixel. In the diffraction simulation the star images on axis and off-axis are nearly identical. In the spot diagram 21mm off-axis the spot size is an incredible 6 microns RMS diameter. This means stars across a 52 mm image circle are going to be pinpoints as small as the atmospheric seeing will allow.


Both of the simulations take into consideration a flat field, which is a more accurate representation of how the optics would perform on a flat CCD camera chip. For visual use some amount of field curvature would be allowed since the eye is able to compensate for a curved field.  The diffraction simulation was calculated at 585nm.  The spot diagram was calculated at 720, 585, and 430nm.  Many companies show spot diagrams in only one wavelength, but you cannot see the chromatic performance with only one wavelength.

Comparison:  CDK vs. Ritchey Chrétien


The simulations shown compares the optical performance of the CDK design to the Ritchey Chrétien (RC) design. The Ritchey design was popularized as an astroimaging telescope due to its use in many professional observatories. Although very difficult and expensive to manufacture and align, the Ritchey is successful in eliminating many of the problems that plague many other designs, namely off-axis coma. However the Ritchey does nothing to eliminate the damaging effects of off-axis astigmatism and field curvature.


The CDK design tackles the off-axis coma problem by integrating a pair of correcting lenses into a two mirror design.  The beauty is that this design also corrects for astigmatism and field curvature. Because the lenses are relatively close to the focal plane (unlike the Schmidt corrector plate found in various Schmidt Cassigrain designs), and because these lenses work together as a doublet, there is no chromatic aberration.  The CDK offers a wide aberration-free, flat field of view that allows the user to take full advantage of the very large imaging chip cameras in the market place today. 


Having an aberration free telescope design means nothing if the optics cannot be aligned properly.  Many Ritchey owners never get to take full advantage of their instrument’s performance because the Ritchey is very difficult to collimate.   Aligning the hyperbolic secondary mirror's optical axis to the optical axis of the primary mirror is critical in the Ritchey design, and the tolerances are unforgiving.  The secondary mirror of the CDK design is spherical.  It has no optical axis and so the centering tolerance of the CDK secondary mirror is comparatively huge.  With the help of some very simple tools, the CDK user will be able to set the secondary spacing, collimate the optics and begin enjoying the full performance potential the instrument has to offer within a few minutes. 


The drastic difference in performance between the CDK and the RC is apparent.  The biggest component that degrades the off-axis performance of the RC is the defocus due to field curvature.  In many diagrams shown by RC manufacturers, the diagrams look better than this because they are showing a curved field.  This is fine for visual use because the eye can compensate for some amount of curvature of field.  But CCD arrays are flat and so in order to evaluate the performance a spot diagrams and/or diffraction simulations requires a flat field as shown.


Aperture:    12.5 inch (318 mm)
Focal Length:     2541 mm (100.04 inch)
Focal Ratio :    f/8
Central Obstruction:     42% of the Primary Mirror Diameter
Back Focus from Mounting Surface:     10.445 inch (265 mm)
Back Focus from Racked in Focuser :    7.2 inch (183 mm)
Weight:     46 lbs (20.9 kg)
OTA Length:   31 inch (787 mm)
Optical Tube:     Carbon Fibre

Optical Performance:  Vignetting and Spot Size Plots
                                           Spot and Diffraction Diagrams
Overall Dimensions:     Overall Dimensions (PDF)

Primary Mirror

Diameter:     13 inch (330 mm)
Aperture:     12.5 inch (318 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;     4.65 inch (118 mm)
Material:     Precision Annealed Pyrex
Shape:     Spherical
Coating:     Enhanced Aluminum - 96%\

Lens Grouping

Diameter:     70 mm (2.76 inch)
Number of lenses:     2
Coating:      Broadband AR Coatings (less than .5% reflected from 400 to 700nm)

2.75" to 2" Adapter (125355) Adapts the 2.75" inner diameter of the focuser to a standard 2" inner diameter. This adapter is necessary when setting the primary to secondary spacing. The 125399 Visual 2.75" 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.
OTA Cover The OTA cover is for closing the front of the tube to protect the primary mirror and the inside of the optical tube.
Printed Instructions For Collimation and Spacing

Instruction Manuals



PlaneWave Interface 3 (PWI 3)

Control software for the Electronic Focuser Assembly (EFA) and Delta T heater.

Note: In order to use this software, the computer must be connected directly to the "PC Port" on the EFA box. If you are using the older method of connecting the computer to the port on the bottom of the handpaddle and would like to upgrade your software to PWI 3, please contact us to arrange a cable upgrade kit.

Note: You are not required to un-install any old versions of PlaneWave Software.

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