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The Planewave Instruments CDK14 Tested and Reviewed

The Planewave CDK14 on an AP1100 mount housed in the NexDome 8ft (2.2m) observatory dome. Credit: Rouzbeh Bidshahri.

Having owned the Planewave CDK12.5 (reviewed for AGT here), I was very interested in how much difference I’d find with the larger CDK14. I was a bit hesitant to take the leap at first, as the two telescopes’ focal lengths are almost identical and the focal ratios very close (f/8 and f/7.2 for the 12.5 and 14, respectively). The 14-inch offers slightly more aperture (12% increase in theoretical resolution, 21% increase in light-gathering power).

But after several months of extensively testing the CDK14, I can now see the differences in design and performance, which I share here for those considering a premium, large-aperture imaging telescope that isn’t too big and heavy to manage in a home observatory.

OTA Structure

The OTA (optical tube assembly) arrived well-padded in a very solid wooden crate. I appreciated the hard plastic covers protecting the primary mirror and rear end of the OTA.

OTA as it arrived, securely packaged. Credit: Rouzbeh Bidshahri.

The OTA feels very rigid and well-built, with structural parts all CNC-machined aluminum. It’s also an open truss tube design, unlike the solid tube of the CDK12.5 design. The carbon fiber truss tubes are very stiff and light, keeping the large OTA’s weight to only 48lbs (22kg).

Several other manufactures use open truss designs, but most either have aluminum lower cages or bolt aluminum dovetails to their carbon fiber lower cages. With the CDK14’s carbon fiber lower cage, its entire length is carbon fiber, which contracts about six times less than aluminum does as temperatures drop. Metal dovetails are connected by flexible joints that don’t affect the length of the OTA with thermal expansion. This is important for holding focus throughout the night as ambient temperatures drop.

The CDK14 comes without a focuser, which you can order separately to suit your needs. The OTA’s backplate includes a lock ring that allows you to choose from several focusers from Planewave and third-party manufacturers.

Carbon Fiber and CNC aluminum construction of the OTA. Note the flexible thermal joint at one end of the dovetail plate. Credit: Rouzbeh Bidshahri.

Collimation

I have used quite a few telescopes of different designs over the years, and the CDK14 is the simplest and most pleasant to collimate.

The primary mirror is fixed, permanently bonded to the rear backplate, making collimation extremely simple. All the user has to do is collimate the secondary.

The spacing between the mirrors is locked in place from the factory so you never need to touch that. Planewave even stopped shipping a Ronchi spacing adjustment kit with the telescope.

That leaves the tip/tilt of the secondary mirror to adjust. The secondary is held in place by a beautifully machined, light, and solid central hub that incorporates the classic 3-screw collimation design, which I prefer to a 4-screw design. You can adjust any screw without needing to loosen opposing screws.

Secondary mirror assembly and 3D printed baffles. Credit: Rouzbeh Bidshahri.

Collimation is very stable thanks to the solid carbon trusses and sturdy spider vanes. My OTA was shipped internationally and was almost perfectly collimated when it arrived. I had to purposely de-collimate it to see how it worked!

The defocused star method gets you collimated in minutes. I achieved perfect collimation using SkyWave collimator (see my review for AGT here). I then confirmed collimation by imaging the Airy disc of a star at using the full 13m focal length.

Collimation and optics checked for on-axis coma using both an image simulated with SkyWave and an actual image of an Airy disc. Credit: Rouzbeh Bidshahri.

Thermal Characteristics

A big advantage of an open truss design is quickly shedding heat, which is even more important with large mirrors. This allows the optics to equalize with the ambient temperature faster. I usually open my dome an hour or two before dark to let things cool down.

The primary mirror’s backside is machined into a conical shape, removing excess material that’s not needed on the outer edges. This reduces both the weight and thermal mass of the mirror.

There are also three mesh-covered fans that blow air onto the rear of the mirror to cool it faster. This airflow also scrubs any air boundary layer that might stick to the mirror. To test for fan vibration, I took 10 test images with the fans on and off and could not detect any increase in star sizes with the fans on.

The open tube design also greatly reduces internal tube currents that would blur images.

Dew Control

Dew can form on the mirrors if they cool too much, dropping below the ambient air temperature. I really like the CDK14 dew control system. It just works!

Sensors monitor the temperature of both mirrors, the ambient air, and the backplate. There are heaters in both mirrors, ready if needed, to ensure that the mirrors don’t cool below the ambient temperature and risk dew formation.

Everything is prewired, and with the optional Delta-T controller you can set a small increment for the mirrors to stay above ambient. The combination of the heat and airflow from the fans has been very effective against dew formation. Imaging from a cold coastal location, I often encounter 90% humidity, but the dew control system has worked every time.

CDK14 OTA rear end showing the internal prewired electronic ports, fans, and Delta-T dew controller. Credit: Rouzbeh Bidshahri.

Optical Design and Performance

The CDK14 is a corrected Dall-Kirkham design (CDK). It uses an elliptical primary mirror and spherical secondary, with the addition of a lens group before the focal plane to correct off-axis aberrations inherent in the Dall-Kirkham design. The CDK14 has a medium-range focal ratio of f/7.2, with a focal length of 2653mm. The secondary mirror creates a 48.5% obstruction.

The two-element corrector at the rear of the telescope is as big as a medium sized refractor’s objective at 95mm (3.7 inches). There is lots of back focus; 11 inches (281.6mm).

Closeup of the rear corrector lens assembly and mirror support. Credit: Rouzbeh Bidshahri.

I really like the fused silica mirrors, which have a very low thermal expansion coefficient, further decreasing focus shift and reducing surface deformation with temperature changes.

Another nice feature is the lightweight 3D printed baffle with complex internal knife edge bafflettes that help keep stray light out of the detector. I also opted for the fabric shroud for further protection from stray light, dust, and bugs while still allowing sufficient ventilation.

Internal view of the 3D printed baffle for hole in the primary mirror. Credit: Rouzbeh Bidshahri.

The optics are excellent, and the design provides both a very large 70mm illuminated field and more importantly keeps the star sizes very small across that field. That’s important as modern sensors get larger while having smaller pixels.

While you can certainly use the telescope at its native focal ratio, Planewave Instruments also offer a reducer (previously reviewed for AGT at astrogeartoday.com/testing-planewave-instruments-0-66x-focal-reducer). I’m using it to capture larger fields of view during nebula season at the resulting fast f/ratio of f/4.75. With the exception of the stars at the very extreme corners, the image quality with a full frame sensor is excellent.

Tests and Results

I spent the first few months with the CDK14 getting some important performance benchmarks before starting any imaging projects.

Illumination and vignetting: A full-frame sensor 43mm across and 50mm round unmounted filters were used here. The light fall-off is only 10% in the extreme corners, which is excellent.

Illumination drop analysis with CCD Inspector. Note the maximum drop of 10% in the extreme corners of a full-frame sensor. Credit: Rouzbeh Bidshahri.

Off-axis star shapes: Using the full-frame sensor with very small 3.76micron pixels and a scale of 0.31 arcseconds/pixel, I was able to capture perfectly round stars in the extreme corners.

Extremes of a single raw exposure showing pinpoint round stars even at the corners of the large 43mm sensor with small 3.76-micron pixels. Credit: Rouzbeh Bidshahri.

Above is a mosaic of the center and corners of an image. Below is a numerical analysis of a frame showing almost identical star sizes across the field (left; nearly equal FWHM). Star roundness (right) is almost perfectly even across the field, with eccentricity values all at about 0.3. Anything below 0.4 is considered very round. 

Star sizes in arcseconds across the field (left) and roundness (right). Credit: Rouzbeh Bidshahri.

 

Field curvature: Below is a 3D plot of the calculated curvature of the focal plane. The 10% curvature result indicates a very flat field.

 

Starfield analyzed with CCD Inspector, showing a very flat field with only 10% curvature over the 43-millimeter width of the field. Credit: Rouzbeh Bidshahri.

I also wanted to ensure that the optics were diffraction limited, as indicated by the Strehl ratio greater than 0.8 promised by Planewave. Not having access to an interferometer for wavefront analysis, or an optical test report, I reached out to Innovations Foresight. They tested the optics with SkyWave Pro, with the following results.

Wavefront analysis results from SkyWave pro, with a Strehl ratio of 0.92. Credit: Innovations Foresight / Dr.Gaston Baudat.

I also tested the optics with the Pixinsight Wavefront Estimator using a series of intra- and extra-focal exposures averaged to compensate for seeing. Results were consistent with SkyWave Pro and other indicators.

Pixinsight Wavefront Estimator indicated a Strehl ratio of 0.846. Credit: Rouzbeh Bidshahri.

Final Thoughts

The CDK14 strikes a fine balance between aperture, weight, price, and focal length. The 48lb (22kg) weight means it can be carried by many mid-sized mounts. I’m also planning to take it to a star party this summer, which would not be possible with larger imaging telescopes.

Perhaps most important of all is the excellent optical performance. The optics are the sharpest I’ve tested, with a massive flat field. Part of the performance can be attributed to the stability afforded by the well-designed mechanical build. Holding collimation and focus simply isn’t a concern with the CDK14.

While there are other large-aperture options available, my choice would be the CDK14 for those looking for a fine instrument that offers observatory-class performance while being practical for use as a backyard telescope as well.

The author’s most recent image with the CDK14, a closeup of the Cygnus wall using narrowband filters. Credit: Rouzbeh Bidshahri.

Full resolution images are available in the author’s gallery: https://www.astrobin.com/users/Rouzbeh/

 

MSRP: $14,500

Website: www.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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