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PlaneWave CDK 17"The PlaneWave Instruments CDK17 is a 17 inch (0.43 m) f/6.8 Corrected Dall-Kirkham Astrograph telescope. The CDK17 covers a 70 mm field of view without any field curvature, off-axis coma, or astigmatism. The instrument weight is 94 lbs (43 kg) and comes standard with a back plate retaining ring ready to accept the focuser of your choice.
The abbreviation CDK stands for the optical system of the PlaneWave astrographs - a Dall Kirkham Cassegrain with a optical correction unit. More about the Optical Design (CDK).
The telescope is available in fused silica optics.
Fused Silicia 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 degree 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.
We presented the CDK 17" together with Rick Hedrick, the CEO of CDK, at the ATT 2008 in Essen.
- Aperture: 17" = 430 mm
- Focal length: 2.940 mm
- Focal ratio: f/6.8
- Obstruction: 49% of the Primary Mirror Diameter
- Field of View = 70 mm (instead of the normal CDK 17" - 52mm)
- Figure = diffraction plates of 6.5 mu / 21mm off axis and 9.6 mu / 26mm off axis at 430, - 585 and 730 nanometer wavelength
The graphic on the left shows the vignetting and spot diagrams for the CDK 17 Widefield Astrographer depending on the large CCD chips KAF 6303, KAF 11000 and KAF 09000 for the three wavelengths 730, 585 and 430 Nanometer.
The diameter of a star at the edge of the flat field corresponds to the pixel size of current large-format CCD chips and the imaging quality of the optics is limited only by the seeing conditions and the quality of the tracking. The large field of view of 70mm diameter offers enough tolerance for CCD cameras with ever larger chips - also for amateur astronomy.
Pictures taken by CDK17":
Specifications:
OPTICAL SYSTEM
Aperture 17 inch (432 mm) Focal Length 2939 mm (115.71 inch) Focal ratio f/6.8 Central Obstruction 23.7% by surface area; 48.6% of the Primary Mirror Diameter Back Focus from Mounting Surface 10.24 inch (260 mm) Back Focus from Racked in Focuser 7.24 inch (184 mm) Weight 106 lbs (48 kg) OTA Length 42 inch (1067 mm) Optical Performance 6.5 micron rms at 21mm and 9.6 micron at 26mm off-axis UpperCage Carbon Fiber Truss Lower Cage Carbon Fiber Truss and Light Shroud Optimal Field of View 70mm Image Circle SECONDARY MIRROR
Diameter 190 mm (7.48 inch) Material Precision Annealed Pyrex Shape Spherical Coating Enhanced Aluminum - 96%
PRIMARY MIRROR
Optical Diameter 17 inch (432 mm) Outer Diameter 17.5 inch (445 mm) Shape Prolate Ellipsoid Material Fused Silica Coating Enhanced Aluminum - 96% LENS GROUP
Diameter 105 mm (4.13 inch) Number of lenses 2 Coating Broadband AR Coatings (less than .5% reflected from 400 to 700nm) STANDARD FEATURES
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 cooling fans blow air inside the back of the telescope. This helps the telescope to reach thermal equilibrium quickly. The fans are controlled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased.
Related Articles
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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 end result is a telescope which is free from off-axis coma, off-axis astigmatism, and curvature of field, yielding a perfectly flat field all the way out to the edge of a 52mm image circle. This means pinpoint stars from the center out to the corner of the field of view. 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 52 to 70mm image plane. Optical Performance At this point, we will show you two diagrams, to convince you about the high image quality of the CDK20. The first...
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Cleaning a CDK17 mirror with Optical Wonder Fluid
In our Observatory installations, we have contact to customers for many years. This is why we get old telescopes for cleaning from time to time. This particular mirror is from a CDK 17" wich was used in a heavily poluted and salty area. The picture show´s that the mirror lost most of it´s reflectivity. The fun fact about this is that after the cleaning our customer wondered why all his images where saturated. The reason was simply that the mirror got so much reflectivety back due to the cleaning, that the instrument was twice as fast as it was before. The cleaning was done with Optical Wonder™ Cleaning Fluid. To clean such a heavily poluted mirror, we lay it flat on the table and put a layer of one way tissues on the surface. This layer we make wet with Optical Wonder™ Cleaning Fluid and let it wet for some hours, so that all pollution softens up. Then we carefully remove the tissues and blow the pollution off the mirror with dry air. After this, we repeat the procedure again, but now we remove the tissues sideways from the mirror with no pressure. This step is repeated as often as necessary to...
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Detailed Customer Review on CDK 17" PlaneWave Astrograph
Dear Mr. Baader, I am happy to comply with your request and share with you my criteria that led to the purchase of the 17 inch telescope from Planewave, and following my experience with this telescope. Since this was a rather complex topic, please forgive the long text and its extent , which certainly clearly exceeds a "normal" customer judgment. Download: detailed review on CDK 17" PlaneWave Astrograph (pdf-file) Decision criterions for the choice of the Planewave astrograph were primarily: robust mechanical construction of the whole telescope, fixed installation and collimation of primary mirror and field corrector at factory, stable secondary mirrorcell, easy adjustment of the secondary mirror, easy control of distance between the main and secondary mirror withRonchi test, good focus stability with slight temperature differences, affordable focal reducer available, a price difference of about 10.000 Euro to a RC telescope from renowned European or international manufacters. To the last point, my following remark: I have not looked into the RC telecopes offered on the amateur market, which are delivered at 16 inches opening and a price below 10,000 euros. Production at this selling price - especially for an RC system - was unimaginable to me. In addition, it...
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The Encke Gap in Saturn’s ring system - imaged with only 17" aperture
On 28 July 2018 around 22:00 UTC we obtained the best image of the planet Saturn so far. It was taken through the 17" Planewave Astrograph of the Rooisand Observatory in Namibia. The imaging camera was a ZWO ASI 290M with 2.9 µm pixel size. The primary focal length of the telescope was extended by a factor of 1.7 with a Baader Q-Turret Barlow lens element, the equivalent focal length was about 5 meters. A total of 3000 single images were taken, of which 10% were stacked. A Baader IR pass filter was used to calm the seeing condition. After processing the raw sum image, a thin black line appeared in the outer ring area, which seemed to be the Encke Gap in the A-ring. It was discovered in the year 1837 by the Berlin astronomer Johann Encke, at this time it was estimated to be 300 km wide. According to measurements taken by the Voyager spacecraft in 1980/81, the width is 330 km. At Saturn's distance in July 2018, these 300 kilometres corresponded to only 0.05 arc seconds. However, the resolution of a 17" telescope at the deep red wavelength of 685 nanometres of the IR pass filter is...
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