PlaneWave CDK 12.5 Fused Silica
Capturing the most stunning astrophotographs possible is something our team is passionate about. From design to manufacture, our goal with the Corrected Dall-Kirkham (CDK) 12.5″ telescope was centered around performance and ease of use. The PlaneWave CDK 12.5 Fused Silica is an incredible breakthrough in telescope technology and produces no off-axis coma and no off-axis astigmatism. Additionally, the PlaneWave CDK 12.5 Fused Silica provides a perfectly flat field so your astrophotographs will have stunning clarity from corner to corner of the image without field curvature degrading the photos. Offering the simplicity of single-mirror collimation, the stray light control of advanced baffles, structural performance created through finite element analysis (FEA), and decades of telescope design experience, the CDK12.5 is an exceptional diffraction-limited telescope. CDK12.5 users can experience pinpoint stars edge-to-edge and a 70 x 70 arcminute field of view when using large camera sensors. When equipment fades into the background and simply performs, the astrophotography experience becomes even more fun and rewarding!
The PlaneWave CDK 12.5 Fused Silica is a 12.5 inch (0.32 m) f/8 Corrected Dall-Kirkham Astrograph telescope. The telescope has a closed carbon fiber tube, with 3 cooling fans ejecting air from the back of the telescope. The PlaneWave CDK 12.5 covers a 52 mm field of view without any field curvature, off-axis coma, or astigmatism. The instrument weight is 21kg and comes standard with the large capacity 2.75 inch Hedrick focuser.
|Carbon Fiber Tube 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 aluminium. The expansion joint allows the aluminium dovetail expand and contract without stressing the carbon fiber lower truss|
|2.75 inch Hedrick Focuser||Heavy duty no-slip focuser. The focus tube runs on 5 bearings and is driven by a leadscrew so there is no chance of slipping. Focus may be automated through a computer using PlaneWave’s EFA Kit add-on. The draw tube travel is 1.3 inch. Image 1 Image 2|
|Cooling Fans||Three fans blow out of the optical tube pulling air though the telescope and by the primary mirror. This helps the telescope to reach thermal equilibrium quickly. The fans are controlled by a switch on the optical tube or can be controlled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased.|
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 52mm image plane.
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 be allowed since the eye is able to compensate for a curved field. The diffraction simulation was calculated at 585nm. The spot diagram was calculated at 720, 585, and 430nm. Many companies show spot diagrams in only one wavelength, but you cannot see the chromatic performance with only one wavelength.
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.
PlaneWave CDK 12.5 Specifications
|Aperture||318mm (12.5 inch)|
|Focal Length||2541 mm (100.04 inch)|
|Central Obstruction||42% of the Primary Mirror Diameter|
|Back Focus from Mounting Surface||265mm (10.445 inch )|
|Back Focus from Racked in Focuser||183mm (7.2 inch)|
|OTA Length||787mm (31 inch)|
|Optical Tube||Carbon Fiber|
|Dimensions||Overall Dimensions (PDF)|
|Weight (includes manual-focuser and dovetail)||20.9 kg (46 lbs)|
|Weight (includes electronic-focuser and dovetail)||22.0 kg (48.5 lbs)|
|Diameter||118 mm (4.65 inch)|
|Material||Precision Annealed Fused Silica|
|Coating||Enhanced Aluminium – 96%|
|Optical Diameter||318 mm (12.5 inch)|
|Outer Diameter||330 mm (13 inch)|
|Material||Precision Annealed Fused Silica|
|Coating||Enhanced Aluminium – 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||73.9 kg|
|Crate Width||559 mm|
|Crate Height||737 mm|
|Crate Length||1,219 mm|
|Motorized 2.75″ Hedrick Focuser||Offers 1.3″ of focusMotorized 2.75″ Hedrick Focuser Offers 1.3″ of focuser travel and takes up 3″ of backfocus. Requires the 125901 EFA kit sold separately.|
|Heating elements for dew prevention||The heating pads on the primary and secondary mirror require the 600195 Delta-T controller sold separately|
|OTA Cover||To protect the primary mirror and inside of the optical tube|
|Flashdrive||Contains software and instructions for collimation and spacing the primary to secondary mirror|
|Cable connector for fan power||Provides a connection method for powering for the fans if the user does not have the 125901 EFA kit. User must provide 12VDC power supply 2.1 barrel jack connector that is center positive.|