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PlaneWave CDK 12.5" f/8 OTA (Optical Tube Assembly) with Fused Silica Optics

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PlaneWave CDK 12.5" f/8 OTA (Optical Tube Assembly) with Fused Silica Optics

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  • CDK12.5 Astrograph, 12.5 inch with Fused Silica Optics
  • modified to Dall-Kirkham, with fully corrected image field
  • 3" Dovetail, focal length 2.541 mm (f/8)
  • Delta T Ready
  • 3.5" Hedrick Focuser (without EFA Kit)

After the successful introduction of the CDK 20" Astrograph from PlaneWave for few years ago, the CDK 12.5" is now available in a smaller and even more affordable aperture. The CDK 12.5" is an Astrograph in the price-range of many amateur-astronomers.

The abbreviation CDK stands for the optical system of the PlaneWave astrographs - a Dall Kirkham Cassegrain with a optical correction unit. The result is the CDK 12.5" which is free from off-axis coma and off-axis astigmatism, yielding a perfectly flat field all the way out to the edge of a 52mm image circle. More about the Optical Design (CDK).

The PlaneWave CDK 12.5 is available in various types of mirror material. Standard mirror material is borosilicate. The telescope is also available in fused silica 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.


At this point, we will show you a spot diagram, to convince you about the high image quality of the CDK 12.5. On the left column, the diagram shows a diffraction simulation for 12.5" CDK optics, and the right column shows the calculated spot diagram. The small squares in the diagram are 9 micrometer (0.009 mm) wide. On-axis is on top, the center line is 15.6 mm off-axis (0.35 degrees) and the bottom line is 26 mm off-axis (0.58 degrees).

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.

If you are working with CCD cameras from SBIG (with or without AO8), accurate focus distances to the CCD chip (spacing of the primary mirror / chip surface) must be met to maintain the planarity of the field and get a good image. For this purpose, there are adapters for different configurations.


SKU (#) 1323212Q
Manufacturer SKU (#) 125103Q
EAN Code 4047825034960
Optical Design Corrected Dall-Kirkham (CDK)
Aperture (mm) 12,5" (318mm)
Focal Length 2.541 mm (100.04 inch)
Focal Ratio f/8
Image Field 52 mm
Back Focus Distance 10.45inch (265 mm) from Mounting Surface, 7,2 inch (183 mm) from Racked in Focuser
Central Obstruction 42% of the Primary Mirror
Substrate Fused Silica
Optical length (mm) 31 inch (787 mm)
Optical Tube Weight 46 lbs (21 kg)


Diameter 4.65 inch (118 mm)
Material Precision Annealed Borosilicate or Fused Silica
Shape Spherical
Coating Enhanced Aluminum - 96%



Optical Diameter 12.5 inch (318 mm)
Outer Diameter 13 inch (330 mm)
Shape Prolate Ellipsoid
Material Precision Annealed Borosilicate or Fused Silica
Coating Enhanced Aluminum - 96%


Diameter 70 mm (2.76 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
2.75 inch Hedrick Focuser Heavy duty no-slip focuser. The focus tube runs on 5 bearings and is driven by a leadscrew so there is no chance of slipping. Focus may be automated through a computer using PlaneWave's EFA Kit add-on. The draw tube travel is 1.3 inch.
Cooling Fans Three fans blow out of the optical tube pulling air though the telescope and by the primary mirror. This helps the telescope to reach thermal equilibrium quickly. The fans are controlled by a switch on the optical tube or can be conrolled 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.

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