Review of the Planewave CDK 12.5 – AstroGear Today
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The author’s Planewave 12.5″ CDK setup. Credit: Rouzbeh Bidshahri

The CDK12.5, or Corrected Dall-Kirkham, is the smallest telescope of the line-up offered by Planewave Instruments currently priced at $10,000. It comes with plenty of aperture and an f/8 focal ratio. It is ready to image out of the box (a wooden crate), except for the controller, which is an additional $920. We’ll take a closer look at the performance, ease of use, and overall quality of the telescope with some examples.

Image Quality

The CDK was designed primarily as an astrograph, i.e., imaging telescope. Its optical design offers a very large image circle free of curvature, coma, or astigmatism. The 12.5-inch aperture is optimized for up to a 52mm diagonal, large enough to accommodate even the largest sensors.

This spot diagram provided by Planewave shows the theoretical size of stars (without atmospheric seeing) in microns. Credit: Planewave

 

Spots and vignetting vs distance off-axis in mm. Credit: Planewave

Here we have a mosaic of the extreme corners of M106 galaxy, captured with the CDK12.5 and an APS-C sized sensor. We can see the stars are sharp are perfectly round in all corners:

The corners of the galaxy M106 imaged with the CDK12.5 and an APS-C sized sensor. Star images are sharp and perfectly round in all the corners. Credit: Rouzbeh Bidshahri

Collimation

Probably the most pleasing aspect of using the CDK12.5 is its ease of collimation. Most astrographs tend to be unforgiving when it comes to collimation, that is, the perfect alignment of the mirrors. For example, Ritchey-Chretien telescopes are notorious for being extremely demanding in this aspect. 

The CDK, on the other hand, has a unique fixed primary mirror and a spherical secondary mirror; it is a matter of simply adjusting the thumb screws on the secondary. This model has four screws, I personally prefer the three-screw collimation system. 

Not only is the CDK very forgiving, but it’s also very rigid and can hold collimation for very long periods of time. I have gone several months since the telescope has been collimated. That’s partly thanks to the new, beefier housing.

The old and new spider and collimation mechanisms on the Planewave CDK 12.5. Credit: Rouzbeh Bidshahri

Focus Stability

Most telescopes need to refocus throughout the session because materials tend to contract with dropping nighttime temperatures. The CDK12.5 tube is constructed out of carbon fiber that has a much lower thermal expansion rate compared to metals typically used (about 6.5x less than aluminum). The mirrors on the newer models are made of fused silica (quartz) that is far more thermally stable than Pyrex.

Expansion joints between the aluminum dovetails and carbon tube allow the metal to contract without straining the tube and altering the focus. Credit: Rouzbeh Bidshahri

Thermal Control

Thermal management is an important factor for larger telescopes. If the primary mirror is unable to shed heat throughout the night, heated air on the surface of the mirror, known as the boundary layer, will blur the image. The CDK primary mirror is also conical, meaning it’s a lot lighter, and has much less heat stored, than conventional cylindrical mirrors. There are three fans that force air past the mirror to keep it as close as possible to the ambient temperature. 

The CDK 12.5 conical primary mirror and three-fan cooling system. Credit: Rouzbeh Bidshahri

We also don’t want the optics to cool below the ambient temperature, as this would cause dew to form. The telescope is equipped with sensors on the mirrors as well as heaters. With the help of the electronics (an optical Delta-T controller), it will detect the ambient temperature and modulate the heaters to keep the mirrors at a slightly higher temperature, thus avoiding dew.

This feature is very effective, and I have yet to experience any dew issues even on nights with 95% relative humidity. The two electronic controllers are a bit bulky and expensive; I would have liked to see a sleeker box with a single USB cable.

Planewave temperature monitoring screen. Credit: Rouzbeh Bidshahri

Build Quality

The telescope is a lot larger than you would expect a 12-inch to be (31 inches or 78 cm, and 49 lbs or 22 kg). The clear aperture is 318mm, and there is a mask over the edges of the primary mirror, which is always preferred for image quality. While the design is simpler than some exotic brands with trusses, it is very practical. The carbon tube has a smooth glossy finish and is matte painted on the inside. It is very effective against dust, stray light, and dew. A plastic dust cap is also included. The optional piggy-back bar makes it a lot easier to handle the telescope.

Metal parts are CNC machined and powder coated. The newer design secondary assembly and spider is extremely rigid.

While the included 2.75-inch focuser is sufficient, upgraded options from Planewave or others would offer more rigidity. I have been able to detect minute amounts of flex and backlash. 

 Final Thoughts

My personal experience with the CDK12.5 has been very positive. In fact after about a year of imaging with it, I’ve placed an order for the larger 14-inch version. The number one contributing factor to my decision was the ease of use and practicality of these telescopes.

While faster f/ratios are usually preferred, this version does have a slightly longer than typical focal ratio of f/8. However, there is a focal reducer to bring it down to f/5.3, if required, but it does add to the cost and complexity.

Melotte 15, a nebula and star cluster in the Heart Nebula, imaged with the Planewave CDK12.5. Credit: Rouzbeh Bidshahri
Galaxy M101 imaged with the Planewave CDK12.5. Credit: Rouzbeh Bidshahri

Long focal length deep-sky astrophotography is challenging enough without having to deal with gear complications. The CDK12.5 has simply worked without any drama, and has enabled me to capture some of my best images yet.

The high-res images can be seen at:  https://www.astrobin.com/users/Rouzbeh/ 

MSRP: $10,000
Website: planewave.com

About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

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