PlaneWave CDK 12.5" f/8 Optical Tube Assembly

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SKU 1323212
  • PlaneWave 12.5" CDK f/8 Optical Tube Assembly
  • Closed carbon fibre tube
  • Fan assisted cooling
  • 52mm image circle with no field curvature, off-axis coma or astigmatism
  • Comes with 3"-wide Losmandy style dovetail

The PlaneWave Instruments CDK12.5 is a 12.5 inch (0.32 m) f/8 Corrected Dall-Kirkham Astrograph telescope and is the smallest and lightest of PlaneWave's OTA offerings weighing only ~21kg (46lbs). This means the telescope can be mounted on medium mounts such as the 10Micron GM2000, Paramount MX+ etc.

PlaneWave CDK12.5" on a Paramount ME

PlaneWave CDK12.5 mounted on a Paramount MEII ready for CCD imaging

The telescope has a closed carbon fibre tube, 3 cooling fans ejecting air from the back of the telescope to cool the telescope's mirrors to help bring the telescope into thermal equilibrium.

The CDK12.5 covers a 52 mm field of view without any field curvature, off-axis coma, or astigmatism. The telescope OTA comes as standard with the large capacity 2.75 inch Hedrick focuser unit.

Telescope Features

THe CDK12.5 comes with the Hedrick focuser - a heavy duty non-slip focuser that is capable of handling instrument payloads of up to 9kg/20lbs.

When using the Hedrick focuser with the optional PlaneWave EFA kit , you can remotely control the focuser using PlaneWave's PWI control PC software. You can download the PWI software from PlaneWave's Downloads page HERE. Alternatively the focuser can be controlled from within the MaximDL software package.

PlaneWave Hedrick Focuser

PlaneWave Hedrick Focuser that is supplied as standard with the CDK12.5.

For dew prevention, the CDK12.5 is "Delta-T ready" meaning it is internally wired with polyimide film heater pads and temperature sensor, ready to be controlled with the optional Delta-T controller.

To help the primary mirror to cool, there are three cooling fans that blow air inside the back of the telescope for even cooling to help the telescope reach thermal equilibrium quickly. The fans are controlled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased.

CDK12.5 Cooling fans
PlaneWave CDK12.5 rear primary cooling fans.
Optical Features

The Ritchey Chrétien (RC) optical design has long been "the standard" in high end imaging telescopes due to its use in many professional observatories. The RC is successful in eliminating many of the problems that plague many other optical designs, namely off-axis coma, however the RC design does nothing to eliminate the damaging effects of off-axis astigmatism and field curvature. The RC design is also very difficult, and therefore expensive, to manufacture and optically align. 

CDK Optical Design

Optical Design of the PlaneWave Corrected Dall-Kirkham (CDK)

The CDK design tackles the "off-axis coma problem" by integrating a pair of correcting lenses into a two mirror design. The beauty is that this design also corrects for astigmatism and field curvature. Because the lenses are relatively close to the focal plane (unlike the Schmidt corrector plate found in various Schmidt Cassegrain designs), and because these lenses work together as a doublet, there is no chromatic aberration. The CDK offers a wide aberration-free, flat field of view that allows the user to take full advantage of the very large imaging chip cameras in the market place today.

PlaneWave CDK12.5 Optical Ray diagram

On the image above, the left shows a diffraction simulation for 12.5" CDK optics, and the right column shows the calculated spot diagram. The small squares are 9 microns wide - typical CCD pixel sensor size. On-axis is on top, the centre 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.

Having an aberration free telescope design means nothing if the optics cannot be aligned properly. Many Ritchey owners never get to take full advantage of their instrument’s performance because the Ritchey is very difficult to collimate. Aligning the hyperbolic secondary mirror’s optical axis to the optical axis of the primary mirror is critical in the Ritchey design, and the tolerances are unforgiving. The secondary mirror of the CDK design is spherical. It has no optical axis and so the centering tolerance of the CDK secondary mirror is comparatively huge. With the help of some very simple tools, the CDK user will be able to set the secondary spacing, collimate the optics and begin enjoying the full performance potential the instrument has to offer within a few minutes.

There are 2 options available for the type of optical glass used in the CDK12.5 telescope: Borosilicate and fused Silica:

Optical glass options



Aperture 12.5 inch (318 mm)
Focal Length 2541 mm (100.04 inch)
Focal ratio f/8
Central Obstruction 42% of the Primary Mirror Diameter
Back Focus from Mounting Surface 10.445 inch (265 mm)
Back Focus from Racked in Focuser 7.2 inch (183 mm)
OTA Length 31 inch (787 mm)
Optical Tube Carbon Fiber
Dimensions Overall Dimensions (PDF)
Weight (includes manual-focuser and dovetail) 46 lbs (20.9 kg)
Weight (includes electronic-focuser and dovetail) 48.5 lbs (22.0 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)



Crated Shipping Weight 163 lbs   (73.9 kg)
Crate Width 22 inch   (559 mm)
Crate Height 29 inch   (737 mm)
Crate Length 48 inch   (1,219 mm)



2.75" to 2" Adapter (125355) Adapts the 2.75" inner diameter of the focuser to a standard 2" inner diameter. This adapter is necessary when setting the primary to secondary spacing. The 125399 Visual 2.75" to 2" adapter is an optional accessory that is designed to space a 2" diagonal for visual use.
Ronchi Spacer (200354) This spacer is used for setting the primary to secondary spacing. It has an 1-1/4 inner diameter and may be used with 1-1/4 oculars for collimation.
Ronchi Ocular This ocular is a Ronchi screen used for setting the primary to secondary spacing.
OTA Cover The OTA cover is for closing the front of the tube to protect the primary mirror and the inside of the optical tube.
Printed Instructions For Collimation and Spacing
12VDC Power Supply (18778) Provides power for the fans (Not included for European Orders)
Wrench Set (5812A35) English Hex Wrenches (European Orders Only)

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