PlaneWave CDK500 20-inch f/6.8 Telescope and L-500 Mount Observatory Telescope System

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£51,750.00
SKU 1323250Q
  • Complete observatory system comprising a CDK20 f/6.8 Astrograph with fused Silica optics and PlaneWave L-500 alt-az mount
  • CDK20 has 70 mm field of view without any field curvature, off-axis coma, or astigmatism
  • Carbon-fibre truss tube design
  • The L-500 mount can hold payload of over 91 kg (200 lbs) for CDK20 with accessories
  • Mount has zero backlash and zero periodic error
  • Integrated high resolution encoders
  • Incredible slew speed up to 50 degrees per second
Introduction

This PlaneWave observatory telescope system comprises

  • PlaneWave CDK20 OTA with fused Silica optics
  • PlaneWave L-500 direct drive alt-az mount

PlaneWave's CDK20 is a 20-inch (0.508 m) aperture f/6.8 telescope that is very well suited for astro-imaging as well as for visual use. The CDK20 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.

At the image plane of the CDK20, will give you pinpoint stars from the centre 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 CDK20 features a dual carbon-fibre truss design, with 3 cooling fans blowing air throughout the back of the telescope. The instrument weighs 63.5kg (140 lbs) and comes as standard with a back plate retaining ring ready to accept the focuser of your choice. The telescope comes with fused Silica optics as standard.

 

The PlaneWave L-500 mount: is an alt-azimuth, single-fork arm style mount that is capable of carrying a total payload of 91kg/200lbs and therefore can easily carry a PlaneWave CDK20 and accessories (e.g. CCD Camera etc). The mount uses a direct drive motor system with high resolution encoders on both the RA and DEC axes that results in zero backlash and periodic error. The mount can slew at speeds from 20 degrees to up to 50 degrees per second!

The mount combines versatility, simplicity and affordability by combining all the technology of PlaneWave's Observatory-class telescopes into a compact and stand-alone mount and is a breakthrough in mount design, specification and features for the price.

Telescope Features

The CDK20 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 Carbon 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 Primary Baffle

3D Printed Primary Baffle

To mount the CDK20 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.

Expansion Joint

Dovetail expansion joint

For dew prevention, the CDK20 is internally wired with polyimide film heater pads and temperature sensor, ready to be controlled with the optional Delta-T controller.

Internal Heater Pad for Primary

Internal heater pads for dew prevention

To help the primary mirror to cool, three cooling fans eject air from the back of the telescope. 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.

CDK Rear Cooling Fan

One of the CDK's cooling fans 

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.

CDK Optical Design

In the image above, are two simulations showing the CDK’s stunning optical performance (CDK20).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. 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.

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. 

Below are a few images (ARP316 top; Crescent nebula bottom) taken with a CDK20 to show how well they perform. 

 

 Crescent Nebula

L-500 Mount

L Series Direct Drive

PlaneWave L Series AZ Direct Drive

The PlaneWave L-500 features Direct Drive motors and on-axis encoders on both the RA and DEC axes. This eliminates the need for gear-based mechanics, thereby eliminating backlash and periodic error. With the high-resolution encoders providing feedback for the direct drive motors, not only will the telescope track without periodic error and backlash, the mount will also counter wind gusts with precise servo feedback.

CDK Ready for imaging

PlaneWave L Series mount with CDK mounted ready for high speed slewing for satellite tracking.

The L-series direct drive motors can move the telescope at speeds from 2 degrees and up to 50 degrees per second. This makes the mount ideal for tracking satellites or just minimising celestial target acquisition time.

L Series Dual Mounting

Dual mounting for PlaneWave L-Series mounts

The PlaneWave 8"-wide dovetail mounting bracket holds telescopes onto the inside of L mounts fork arm. An additional dovetail clamp is available to allow the mounting of an extra supplementary telescope on the outside of the fork arm adding to the mount's versatility. 

L Series Balancing System

Azimuth horizontal balancing system

Adding extra accessories to a single-mounted telescope, or adding a second telescope can affect the overal balance of the whole telescope. The upper portion of the L mount can be moved horizontally for precise centre-of-gravity balancing whether the mount is in Alt-Az or Equatorial configuration.

L Series Internal Cabling

Internal through the mount cabling

To keep cabling from e.g. CCD cameras, filter wheels etc, the L- series mounts feature interior "through-the-mount" cable routing via access panels in the fork arm and azimuth axis sections.

CDK's at work

PlaneWave CDK700's at work

The PlaneWave alt-az L-Series uses the same proven technology as used in the well-respected CDK700 telescopes which are being used in Universities such as the UK's University of Central Lancashire Alson Observatory >>HERE<<.

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)
Upper Cage Carbon Fibre Truss
Lower Cage Carbon Fibre Truss with Carbon Fibre Light Shroud

 

SECONDARY MIRROR

Diameter 7.5 inch (191mm)
Material Fused Silica
Shape Spherical
Coating Enhanced Aluminum - 96%

 

PRIMARY MIRROR

Optical Diameter 20 inch (508 mm)
Outer Diameter 20.5 inch (521 mm)
Shape Prolate Ellipsoid
Material Fused SIlica
Coating Enhanced Aluminum - 96%

 

LENS GROUP

Diameter 90 mm (3.54 inch)
Number of lenses 2
Coating Broadband AR Coatings (less than .5% reflected from 400 to 700nm)

 


MOUNT SYSTEM

Type Alt-Azimuth / Equatorial Direct Drive Mount
Weight 257 lbs (100 kg)
Component Weights AZ base: 55kg (121lbs) ; Fork arm 62kg (136lbs)
Max. Load Capacity 200 lbs (91 kg)
Latitude Range 0 to 90 degrees, Northern and Southern hemispheres
Cable Management Equipment cables can be wired through mount


CONTROL SYSTEM

Control Electronics PlaneWave Interface dual axis telescope control
User Interface PlaneWave Interface 4 (PWI4) Control Software with integrated PointXP mount modeling software
Homing Sensors Home position sensors are included allowing the mount can find its home position on power up.
Slew Rate 20 degrees per second (standard); 50 degrees per second (maximum), both axes
Power Requirement Accepts 120 VAC. Supplied with 12VDC 15A Regulated Power Adapter


MOTION CONTROL

Motor Control Industrial grade brushless motor control system and built in electronics
Motor - Azimuth and Altitude Direct Drive 3 Phase Axial-Flux Torque Motors
Encoder - Azimuth and Altitude 152mm disk built into the azimuth and altitude axes with stainless steel encoder on the circumference with reader yields 18,880,000 counts per revolution of the telescope. This translates to about 0.069 arcsecond resolution
Motor Torque Approximately 20 ft-lbs continuous; 50 ft-lbs peak
Drive Electronics Industrial grade, off-the-shelf brushless motor drives for each axis with custom designed interface card
Telescope Control Software PlaneWave Interface (PWI4). Incorporates PointXP mount modeling software by Dave Rowe All ASCOM compatible.

 

SYSTEM PERFORMANCE

Pointing Accuracy <10 arcsecond RMS with PointXP Model
Pointing Precision 2 arcsecond
Tracking Accuracy < .3 arcsecond error over 5 minute period
System Natural Frequency 10 Hz or greater

 

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