PlaneWave CDK700 0.7m f6.5 Nasmyth Astrograph Observatory Telescope System

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  • 0,7m f/6.5 CDK optical system
  • Dual Nasmyth focus, allowing simultaneous mounting of two instruments or easy switching between visual and imaging use
  • Pinpoint stars out to a 70mm image circle
  • Ultra-low thermal expansion fused silica optics
  • CDK700 mount uses direct-drive motors on each axis for zero backlash and zero periodic error
  • High resolution encoders on each axis for precise positioning
  • PointXP mount modeling software
  • Field de-rotator / rotational field framing
Introduction

The PlaneWave CDK700 is a 0.7m fully automated observatory Alt-Az class telescope system for serious research.

PlaneWave CD700 being installed at the Alston Observatory at the University of Central Lancashire UK

PlaneWave are combining the revolutionary optical system that founded PlaneWave Instruments with a brand new innovative alt/az mount technology that sets a new standard for small professional observatory telescopes. 

CDK700 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.

In the image above, are two simulations showing the CDK700’s stunning optical performance.The left column is a spot diagram and the right column is a diffraction simulation. In both of the simulations the small squares are 9x9 microns, about the size of a typical CCD pixel. In the spot diagram 21mm off-axis the spot size is an incredible 4.4 microns RMS diameter. For 35mm off-axis, a 70mm image circle, the RMS spot size is 6.8microns. 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.

 

CDK700 Key Features

 

PlaneWave CDK700 Schematic

CDK700 Schematic

The Nasmyth focus position is on the altitude axis which means that there is virtually no balancing issue when you add or change equipment (e.g. eyepieces or CCD camera/filter wheel etc). Furthermore, if the telescope is used visually, the eyepieces remain at a constant, and wheelchair-accessible, height. This simplifying access to the telescope for public observatory events.

CDK700 Nasmyth Focus with a Televue Eyepiece for visual use
Nasymth focus position showing a Televue eyepiece for visual observing. 

Inside the telescope OTA, there is a rotating tertiary mirror that allows you to switch from one Nasmyth port to the other in just seconds, allowing observers to switch between eyepieces or imagers to easily transition between different imaging equipment. 

CDK700 Tertiary Mirror for CDK700

Internal Tertiary Mirror for Nasymth focus switching.

The PlaneWave direct drive motors mean that there are no gears to cause backlash and/or periodic error. With high-resolution encoders on both axes providing the feedback for the direct drive motors, not only will the telescope track without periodic error or have any backlash at all, but the mount will be able to counter against any wind gusts. The direct drive motors can move the telescope at incredible speeds for tracking satellites or just to minimise target acquisition time.

CDK700 Direct Drive

PlaneWave CDK700 altitude direct drive system

Specifications

OPTICAL SYSTEM

Optical Design Corrected Dall-Kirkham (CDK)
Aperture 700 mm (27.56 inch)
Focal Length 4540 mm
Focal ratio 6.5
Central Obstruction 47% of the Primary Mirror Diameter
Back Focus 309 mm (12.2 inch) from Mounting Surface
Weight 1,200 lbs
Optical Tube Dual truss structure with Nasmyth focus
Optical Performance 1.8 micron RMS spots [25mm on-axis] (400 to 900nm) 4.4 micron RMS spots [21mm off-axis] (400 to 900nm) 6.8 micron RMS spots [35mm off-axis] (400 to 900nm) Spot Diagram  
Dimensions 93.73" H x 43.25" W x 39" D    
Optimal Field of View 70 mm (0.86 degrees)
Focus Position Nasmyth Focus
Fully Baffled Field 60 mm
Image Scale 22 microns per arcsecond

 

MECHANICAL STRUCTURE

Fork Assembly Single piece U shaped fork arm assembly for maximum stiffness
Azimuth Bearing 20 inch diameter thrust bearing
Altitude Bearing 2 x 8.5 inch OD ball bearings
Optical Tube Dual truss structure with Nasmyth focus

 

MOTION CONTROL

Motor Control Industrial grade Parker brushless motor control system and built in electronics
Azimuth Motor Direct Drive 3 Phase Axial-Flux Torque Motor
Altitude Motor Direct Drive 3 Phase Axial-Flux Torque Motor
Encoder - Azimuth and Altitude 10 inch disk built into the azimuth and altitude axes with stainless steel encoder tape on the circumference with reader yields 16 million counts per revolution of the telescope. This translates to about 0.08 arcsecond resolution.
Motor Torque Approximately 30 ft-lbs continuous; 60 ft-lbs peak
Drive Electronics Capable of controlling up to 4 high speed encoders, limit switch inputs, homing switch inputs, controls two additional motors for accessories, two brake outputs and 16 digital and 2 analog inputs
Telescope Control Software 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 <1 arcsecond error over 10 minute period
System Natural Frequency 10 Hz or greater
Field De-Rotator Accuracy 3 microns of peak to peak error at 35mm off-axis over 1 hour of tracking (18 arc sec)

 

PRIMARY MIRROR

Radius of Curvature 4115mm
Optical Diameter 700 mm (27.56 inch)
Outer Diameter 714.38 mm (28.125 inch)
Core Diameter 203mm (8 inch)
Material Fused Silica
Edge Thickness 2.25 inch
Cell 18 point

 

SECONDARY MIRROR

Radius of curvature 2961 mm
Thickness 25 mm
Optical Diameter 312.4 mm (12.3 inch)
Outer Diameter 317.5 mm (12.5 inch)
Material Fused Silica
Cell 3 point RTV

 

TERTIARY MIRROR

Optical Major Diameter 152.4 mm
Thickness 25 mm
Material Fused Silica

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