PlaneWave CDK 17" Telescope Optical Tube Assembly

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£22,480.00
SKU 1323217W
  • A 17"/0.43m visual and astrographic telescope
  • Uses PlaneWave's Corrected Dall-Kirkham (CDK) optical design
  • 70mm diameter imaging circle
  • No field curvature, off-axis coma, or astigmatism.
  • Carbon fibre truss-tube for low thermal expansion, rigidity and weight saving.
  • Two optical glass options available
Introduction

PlaneWave's CDK17 is a 17 inch (0.43 m) aperture f/6.8 telescope that is very well suited for astro-imaging as well as for visual use.

CDK17 mounted on a GEM with a CCD Camera attached

The CDK17 telescope utilises the Corrected Dall-Kirkham (CDK) optical design, developed by Dave Rowe, that provides excellent imaging possibilities with large format CCD cameras, while remaining superb for visual use. The CDK design far exceeds the off-axis performance of most commercial telescope designs including the Ritchey-Chrétien design making the CDK an innovative solution for unsurpassed astro-imaging quality at an affordable price. 

The end result, at the image plane of the CDK17, will give you pinpoint stars from the center to the corner of its 70mm field of view without any field curvature, off-axis coma, or astigmatism. The no-compromise optical design is unique in making the optical alignment forgiving and collimation very easy which guarantees the user the best possible performance from the telescope. 

The CDK17 features a dual carbon-fibre truss design, with 3 cooling fans ejecting air from the back of the telescope, and 4 fans blowing across the boundary layer of the mirror's surface. The instrument weighs 43kg (94 lbs) and comes as standard with a back plate retaining ring ready to accept the focuser of your choice. 

Telescope Features

The CDK17 telescope is an open truss tube design for a rigid, but lightweight structure, and for quick thermal cooling. The truss tube utilises a carbon fibre frame which minimises thermal expansion that can cause focus shift with changes in temperature. The carbon fibre truss tube design also promotes quick thermal cooling.

CDK Cabron Fibre Truss Poles

The CDK's carbon fibre truss poles 

To create the telescope's baffles, Planewave uses digital 3D printing technology. Their 3D printers add successive layers of material to construct a baffle system with precision. These internal stray light baffles minimise vignetting and maximise image contrast.

CDK 3D Printed Baffle

3D Printed Primary Baffle

To mount the CDK17 onto a mount, PlaneWave supplies the CDK with a dovetail bar. The CDK features a dovetail expansion joint that allows for the difference in thermal expansion between carbon fibre and aluminium. This expansion joint allows the aluminium dovetail expand and contract without stressing the carbon fibre lower truss.

CDK Dovetail Expansion Joint

Dovetail expansion joint

For dew prevention, the CDK17 is internally wired with polyimide film heater pads and temperature sensor, ready to be controlled with the optional Delta-T controller.

CDK Internal Heater Pads for Primary - making it Delta-T ready

Internal heater pads for dew prevention

To help the primary mirror to cool, three cooling fans eject air from the back of the telescope and four fans blowing across the boundary layer of the mirror surface. This helps the telescope to reach thermal equilibrium quickly. The fans can be controlled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased.

CDK17 Cooling Fan

One of the CDK's 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. 

Corrected Dall Kirkham Optical Design Diagram

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.

CDK Optical Performance

In the image above, are two simulations showing the CDK’s stunning optical performance.The left column is a diffraction simulation and the second in the right column is a spot diagram. In both simulations the small squares are 9×9 microns, about the size of a typical 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 stars across a 52 mm image circle are going to be pinpoints as small as the atmospheric seeing will allow. 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.

The diagram above  shows the vignetting and spot diagrams for the CDK 17W for CCD cameras that use the large CCD sensors KAF 6303, KAF 11000 and KAF 09000 for the three wavelengths 730, 585 and 430nm. 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 increasing sized CCD sensors.

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 CDK17 telescope: Borosilicate and fused Silica:

Below are a few images (M81 top; Veil nebula bottom) taken with a CDK17 to show how well they perform. 

M 81

NGC 6960, Veil Nebula

Specifications

OPTICAL SYSTEM

Optical Design Corrected Dall-Kirkham
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
Upper Cage Carbon Fiber Truss
Lower Cage Carbon Fiber Truss with Carbon Fiber Light Shroud
Optimal Field of View 70mm Image Circle


SECONDARY MIRROR

Diameter 190 mm (7.48 inch)
Material Precision Annealed Pyrex
Shape Spherical
Coating Enhanced Aluminum - 96%


PRIMARY MIRROR

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%


LENS GROUP

Diameter 105 mm (4.13 inch)
Number of lenses 2
Coating Broadband AR Coatings (less than .5% reflected from 400 to 700nm)

 


STANDARD FEATURES

Carbon Fiber Truss 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 ejecting air from the back of the telescope and four fans blowing across the boundary layer of the mirror surface. 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.


SHIPPING

Crated Shipping Weight 225 lbs   (102.1 kg)
Crate Width 30 inch   (762 mm)
Crate Height 37 inch   (940 mm)
Crate Length 58 inch   (1,473 mm)

 

INCLUDED ACCESSORIES

Ronchi Spacer (600341 & 600343) This spacer sits in place of a focuser and 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
Primary Mirror Cover The Primary Mirror Cover protects the primary mirror
Planewave Thumb Drive Containing all documentation and software for collimation and spacing
12VDC Power Supply Provides power for the fans (Not included for European Orders)
Wrench Set (5812A35) English Hex Wrenches (European Orders Only)

Downloads

Drawings & Diagrams

 

Camera Adapters Diagrams

Software

Instructions

Helpful Tools

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