PlaneWave CDK 20" Telescope Optical Tube Assembly
- A 20"/0.508m f/6.8 visual and astrographic telescope
- Uses PlaneWave's Corrected Dall-Kirkham (CDK) optical design
- 70mm diameter imaging circle
- No field curvature, off-axis coma, or astigmatism.
- Carbon fibre truss-tube for low thermal expansion, rigidity and weight saving.
- Two optical glass options available
PlaneWave's CDK20 is a 20 inch (0.508 m) aperture f/6.8 telescope that is very well suited for astro-imaging as well as for visual use.
The CDK20 telescope utilises the Corrected Dall-Kirkham (CDK) optical design, developed by Dave Rowe, that provides excellent imaging possibilities 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 making the CDK an innovative solution for unsurpassed astro-imaging quality at an affordable price.
The end result, at the image plane of the CDK20, will give you pinpoint stars from the center to the corner of its 70mm field of view without any field curvature, off-axis coma, or astigmatism. The no-compromise optical design is unique in making the optical alignment forgiving and collimation very easy which guarantees the user the best possible performance from the telescope.
The CDK20 features a dual carbon-fibre truss design, with 3 cooling fans to help cooling of the primary mirror. The instrument weighs 63.5kg (140lbs) and comes as standard with a back plate retaining ring ready to accept the focuser of your choice.
The CDK20 telescope is an open truss tube design for a rigid, but lightweight structure. The truss tube utilises a carbon fibre frame which minimises thermal expansion that can cause focus shift with changes in temperature. The carbon fibre truss tube design also promotes quick thermal cooling.
To create the telescope's baffles, Planewave uses digital 3D printing technology. Their 3D printers add successive layers of material to construct a baffle system with precision. These internal stray light baffles minimise vignetting and maximise image contrast.
To mount the CDK20 onto a mount, PlaneWave supplies the CDK with a dovetail bar. The CDK features a dovetail expansion joint that allows for the difference in thermal expansion between carbon fibre and aluminium. This expansion joint allows the aluminium dovetail expand and contract without stressing the carbon fibre lower truss.
For dew prevention, the CDK20 is internally wired with polyimide film heater pads and temperature sensor, ready to be controlled with the optional Delta-T controller.
To help the primary mirror to cool, three cooling fans eject air from the back of the telescope and helps the telescope to reach thermal equilibrium quickly. The fans can be controlled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased.
The Ritchey Chrétien (RC) optical design has long been "the standard" in high end imaging telescopes due to its use in many professional observatories. The RC is successful in eliminating many of the problems that plague many other optical designs, namely off-axis coma, however the RC design does nothing to eliminate the damaging effects of off-axis astigmatism and field curvature. The RC design is also very difficult, and therefore expensive, to manufacture and optically align.
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 market place today.
In the image above, are two simulations showing the CDK’s stunning optical performance (CDK20).The left column is a diffraction simulation and the second in the right column is a spot diagram. In both of the simulations the small squares are 9x9 microns, about the size of a typical 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. For 26mm off-axis, a 52mm image circle, the RMS spot size is 11 microns. This means the stars across the entire focal plane are going to be pinpoints as small as atmospheric seeing will allow. The spot diagram was calculated at 720, 585, and 430nm (many companies show spot diagrams in only one wavelength, however to evaluate chromatic performance multiple wavelengths are required). 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.
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.
There are 2 options available for the type of optical glass used in the CDK20 telescope: Borosilicate and fused Silica (see below) plus a fused Silica optic-only option for fork mounts
Below are a few images (ARP316 top; Crescent nebula bottom) taken with a CDK20 to show how well they perform.
|Aperture||20 inch (508 mm)|
|Focal Length||3454 mm (135.98 inch)|
|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|
|Diameter||7.5 inch (191mm)|
|Coating||Enhanced Aluminum - 96%|
|Optical Diameter||20 inch (508 mm)|
|Outer Diameter||20.5 inch (521 mm)|
|Coating||Enhanced Aluminum - 96%|
|Diameter||90 mm (3.54 inch)|
|Number of lenses||2|
|Coating||Broadband AR Coatings (less than .5% reflected from 400 to 700nm)|
|Crated Shipping Weight||291 lbs (132.0 kg)|
|Crate Width||33 inch (838 mm)|
|Crate Height||40 inch (1,016 mm)|
|Crate Length||65 inch (1,651 mm)|
|Ronchi Spacer (200354)||This spacer sits in place of a focuser and is used for setting the primary to secondary spacing. It has an 1 .25” inner diameter and may be used with 1.25” 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.|
|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)|
Drawings & Diagrams
- CDK17 Overall Dimensions (2475)
- CDK17 Vignetting and Spot Size Plots (2018)
- CDK17 Bottom Dovetail Plate Diagram (170319) (1858)
Camera Adapters Diagrams
- Popular Spacer Combinations (2890)
- Adapters and Spacers for QSI Cameras (2249)
- Adapters and Spacers for DSLR Cameras (3128)
- CDK17 IRF90 to Apogee Adapters (1731)
- CDK17 IRF90 to SBIG STX Adapters (2022)
- CDK17 to Apogee Adapters (1974)
- CDK17 to FLI Adapters (1843)
- CDK17 to SBIG Adapters (2503)
- Collimation and Spacing Instructions for CDK12.5, 14, 17, 20, and 24 (4207)
- Cleaning Optics CDK14/17/20/24 (2807)