Optical Design

The CDK Optical Design

The CDK [Corrected Dall-Kirkham] telescope is based on a new optical design developed by Dave Rowe. The goal of the design is to make an affordable astrographic telescope with a large enough imaging plane to take advantage of the large format CCD cameras of today. Most telescope images degrade as you move off-axis from either coma, off-axis astigmatism, or field curvature. The CDK design suffers from none of these problems. The CDK is coma free, has no off-axis astigmatism, and has a flat field. The design is a simple and elegant solution to the problems posed above. The CDK consists of three components: an ellipsoidal primary mirror, a spherical secondary mirror and a lens group. All these components are optimized to work in concert in order to create superb pinpoint stars across the entire 52 to 70mm image plane.

Optical Performance

Shown are two simulations showing the CDK’s stunning performance. The first is a diffraction simulation and the second is a spot diagram. In both simulations the small squares are 9×9 microns, about the size of a 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. For visual use some amount of field curvature would

Comparison: CDK vs. Ritchey Chrétien

The simulations shown compares the optical performance of the CDK design to the Ritchey Chrétien (RC) design. The Ritchey design was popularized as an astroimaging telescope due to its use in many professional observatories. Although very difficult and expensive to manufacture and align, the Ritchey is successful in eliminating many of the problems that plague many other designs, namely off-axis coma. However the Ritchey does nothing to eliminate the damaging effects of off-axis astigmatism and field curvature.

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.

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.

The drastic difference in performance between the CDK and the RC is apparent. The biggest component that degrades the off-axis performance of the RC is the defocus due to field curvature. In many diagrams shown by RC manufacturers, the diagrams look better than this because they are showing a curved field. This is fine for visual use because the eye can compensate for some amount of curvature of field. But CCD arrays are flat and so in order to evaluate the performance a spot diagrams and/or diffraction simulations requires a flat field as shown.

Mechanical Design

High Resolution Axes Encoders

PlaneWave’s A200 German Equatorial Mount comes standard with high-resolution Axis Encoders on both Right Ascension and Declination axes.

The A200 encoder technology is a breakthrough in value for a telescope mount using quality components usually founds in professional equipment costing thousands more.

The A200 uses a non-contact encoder design that offers high speed, reliable operation with zero friction and zero wear.

The encoders sits above a 20 µm thin flexible steel strip, which is gold plated to give high reflectivity and corrosion resistance.  This ensured repeatable, precise results for the  life of your mount.

Direct Drive Motors

For added precision and accuracy the CDK700 not only uses high resolution encoders, but also employs a Direct Drive motor system.   Direct Drive motors means that there are no gears to cause backlash or periodic error while slewing and tracking. With the high resolution encoders 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 wind gusts. The direct drive motors can move the telescope at incredible speeds for tracking satellites or just to minimize target acquisition time.

The Direct Drive system is composed of 24 coils and 32 neodymium magnets powered by a 3-Phase Axial-Flux Torque Motor.

Combined with a hi-res encoder and stainless steel encoder tape on the circumference, the drive yields 16 million counts per revolution, or  about 0.08 arc-second resolution.


PlaneWave Interface Software

Behind every great telescope and mount lies a great control program that keeps everything running in Sync. PlaneWave Interface (PWI3) is the one software you will need to monitor and control you CDK inside and out.


PWI3 gives you remote control of you Hedrick focuser, rotating focuser (IRF90), on-board fans and internal dew heaters (optional Delta-T) as well as continually monitoring internal and external temperature sensors.

Now you can focus your focuser, rotate your instrumentation to find a guide star, set your dew heater to turn on at a desired temperature, all from the comfort of your observatory control room.

No matter if your telescope is in your backyard or being controlled remotely from 1,000 miles away, PWI3 will keep you in control.