PLANE WAVE CDK 20", F/6.8, PLANE WAVE DOVETAIL FOR MI750 MT., 140LBS, 200103
Item No. 606-20100
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Manuf. No. PlaneWave 200103
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PlaneWave CDK20 Dovetail Telescope for MI750 Mount, 200103
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-Chrętien 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 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.
Features and Specifications
- 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: 135 lbs
- OTA Length: 47"
- Upper Cage: Carbon Fiber Truss
- Lower Cage: Carbon Fiber Truss with Carbon Fiber light shroud
- Performance: Optical Performance - Vignetting and Spot Performance
- Dimensions Overall Dimensions
- 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%
- Diameter: 7.5"
- Material: Precision Annealed Pyrex
- Shape: Spherical
- Coating: Enhanced Aluminum - 96%
- Diameter: 90mm
- Number of lenses: 2
- Coating: Broadband AR Coatings (less than .5% from 400 to 700nm)
Shipping Dimensions and Weight
- 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 PlaneWaveęs EFA Kit to control many electronic accessories. The draw tube travel is 1.3". (View picture) (View picture2) Drawing of flange - Drawing of draw tube
- 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.
- Crate Dimensions: 59" X 32" X 36"
- Crate Shipping Weight: 291 lbs
The CDK telescope is a brand 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.
Below 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 Chrętien the high end arket of astrographic
The simulations below compares the optical performance of the CDK design to the Ritchey Chrętien (RC) design. The Ritchey design was popularized as an astroimaging telescope due to its use in many professional obsveratories. 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 instrumentęs 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.
In the comparison below 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.