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 PlaneWave CDK17 is available in various types of mirror material. Standard mirror material is borosilicate. The telescope is also 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":
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
Diameter 190 mm (7.48 inch) Material Precision Annealed Pyrex Shape Spherical Coating Enhanced Aluminum - 96%
Optical Diameter 17 inch (432 mm) Outer Diameter 17.5 inch (445 mm) Shape Prolate Ellipsoid Material Precision Annealed Borosilicate or Fused Silica Coating Enhanced Aluminum - 96%
Diameter 105 mm (4.13 inch) Number of lenses 2 Coating Broadband AR Coatings (less than .5% reflected from 400 to 700nm)
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.
|Optical Design||Corrected Dall-Kirkham (CDK)|
|Back Focus Distance||10.24" (260 mm) from Mounting Surface, 7.24" (184 mm) from Racked in Focuser|
|Aperture (mm)||17" (432 mm)|
|Focal Length||2.939 mm (115.71 inch)|
|Image Field||70 mm|
|Central Obstruction||23.7 % by surface area, 49% of the Primary Mirror Diameter|
|Optical tube length||42 inch (1.067 mm)|
|Optical Tube Weight Copy||106 lbs (48 kg)|