Planewave 20” CDK Corrected Dall-Kirkham OTA
with Fused Silica Mirror Upgrade
About Fused Silica…
Fused silica, also known as fused quartz, 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 (about 1600 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.
About the Planewave 20" CDK Telescope...
Created to meet the demands of both the serious imager and visual observer, PlaneWave CDK, a revolutionary new telescope optical system invented by Dave Rowe, is offered at an unprecedented value for a telescope of this quality and aperture. The goal of the design was to make an affordable astrographic telescope with a large enough imaging plane to take advantage of the large format CCD cameras of today. The CDK design far exceeds the off-axis performance of most commercial telescope designs, including the Ritchey-Chrétien. 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. 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.
The PlaneWave CDK Optical Design...
One of the unique features of the CDK design is its ease of collimation and achievable centering tolerance for a telescope of its class. This ease of alignment and collimation guarantees the user will be sure to get the best performance out of the telescope possible, each and every night. The end result at the image plane of the CDK design is pinpoint stars from the center of the field of view out to the corner of the field of view.
PlaneWave CDK Optical Performance...
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 42mm 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-Chretien...
The simulations below compares the optical performance of the CDK design to the Ritchey Chrétien (RC) design. The Ritchey design was popularized as an astro-imaging 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-Cassegrain 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 marketplace 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.
In the comparison shown above 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.
PlaneWave 20" CDK Features...
- 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 to expand and contract without stressing the carbon fiber lower truss
- 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 PlaneWave’s EFA Kit to control many electronic accessories. The draw tube travel is 1.3".
- 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.
- This is a custom telescope and delivery times may vary. Contact us for an ETA before placing your order, or we will contact you afterwards with that information.
- PlaneWave requires a 50% deposit at the time your order is placed. If you decide to cancel that order, the manufacturer will keep $800 from your deposit.
- This telescope comes with a dovetail. Please let us know the mount you will be using the CDK 20" on so that we can get you the proper one.
- We also need to know which CCD camera you will be using so that we can inform you of the proper adapters for your set-up.
- Shipping charges will be quoted separately. Just contact OPT for a shipping quote. The quote will include a shipping crate.
- Additional Information
SKU PI-20101Q Manufacturer PlaneWave Telescope Series Planewave CDK Optical Design Corrected Dall-Kirkham Mount Type None - Optical Tube Only Warranty 2 Year Warranty Telescope Aperture 20" Telescope Focal Length 3454mm Telescope Focal Ratio f/6.8 Optical Coatings Aluminum Tube Color or Finish Carbon Fiber Optical Tube Outer Diameter 521mm Length of Optical Tube 59" Optical Tube Weight 140 lbs. Limiting Stellar Magnitude 16 Highest Useful Magnification Not provided by manufacturer OTA Mount Type N/A Finder Included None Focuser Size 3.5" Power Adapter Included DC Cable with Alligator Clips
- Included Items
- PlaneWave Instruments 20" CDK Corrected Dall-Kirkham Carbon Fiber Truss Telescope
- Upgraded Fused Quartz Mirror
- 3.5" Hendrick Focuser
- 3.5" to 2" Adapter (200349)
- Ronchi Spacer (200354)
- Ronchi Ocular
- Primary Mirror Cover
- Printed Instructions
- Questions & Answers
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