PlaneWave CDK 20"
The 20" CDK is a revolutionary new telescope optical system that is designed to excel at imaging on large format CCD cameras while remaining an excellent instrument for visual use.
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 assures the user will get the best possible performance out of the telescope. 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.
The PlaneWave CDK 20 is available in various types of mirror material. Standard mirror material is borosilicate. The telescope is also available in fused silica optics, Ritchey-Chrétien (RC) optics and infrared optimized.
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.
To the left 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. 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.
New "state of the art" observatory at the University of Leicester
On December 7, 2009, a Planewave CDK 20, mounted on a Paramount, ME was inaugurated at the University of Leicester, Department of Physics and Astronomy. The complete installation and delivery took place by the company Baader Planetarium, Mammendorf.
Pictures taken by CDK20":
Aperture 20 inch (508 mm) Focal Length 3454 mm(135.98 inch) Focal ratio f/6.8 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) UpperCage Carbon Fiber Truss Lower Cage Carbon Fiber Truss with Carbon Fiber Light Shroud Optimal Field of View 52mm Image Circle
Diameter 7.5 inch (191mm) Material Precision Annealed Borosilicate Shape Spherical Coating Enhanced Aluminum - 96%
Optical Diameter 20 inch (508 mm) Outer Diameter 20.5 inch (521 mm) Shape Prolate Ellipsoid Material Precision Annealed Borosilicate or Fused Silica 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)
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. Delta-T Ready For added dew prevention, the CDK14 is internally wired with polyimide film heater pads and temperature sensor, ready to be controlled with the optional Delta-T controller.
|Optical Design||Corrected Dall-Kirkham (CDK)|
|Back Focus Distance||8.8" (223mm)Back Focus from Mounting Surface, 5.8" (147 mm) from Racked in Focuser|
|Aperture (mm)||20" (508 mm)|
|Focal Length||3.454 mm (135.98 inch)|
|Image Field||52 mm|
|Central Obstruction||39% of the Primary Mirror Diameter|
|Optical tube length||47 inch (1.194 mm)|
|Optical Tube Weight Copy||140 lbs (64 kg)|