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PlaneWave RC20"
The PlaneWave Instruments PW20-RC is a 20 inch (0.51 m) f/6.8 Ritchey-Chrétien (RC) Astrograph telescope. The instrument weight is 140 lbs (64 kg) and comes standard with a back plate retaining ring ready to accept the focuser of your choice.
PlaneWave's CDK astrographs have become the telescopes of choice for universities, industry, and astrophotographers around the world, and now PlaneWave offers the PW-RC series to meet the custom needs of the scientific community. More about the Optical Design.
The RC20 in its native format (no field flattening lenses) uses only a primary and secondary mirror with reflective coatings. This can be useful when making observations primarily on-axis and wide field imaging is not a concern.
The PlaneWave RC20 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.
Specifications:
OPTICAL SYSTEM
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 SECONDARY MIRROR
Diameter 7.5 inch (191mm) Material Precision Annealed Borosilicate Shape Hyperbolic Coating Broadband AR Coatings (less than .5% reflected from 400 to 700nm)
PRIMARY MIRROR
Optical Diameter 20 inch (508 mm) Outer Diameter 20.5 inch (521 mm) Shape Hyperbolic Material Precision Annealed Borosilicate or Fused Silica Coating Enhanced Aluminum - 96% 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.
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The CDK Optical Design
CDK 20" Astrograph 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...
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