First Light Review Planewave CDK17

by Johannes Schedler / Oct 2008


Baader Planetarium offered me the opportunity to test the first Planewave CDK17 (aperture 430 mm) in Europe.
I picked up at their company the CDK17 together with a GM2000 mount and accessories like counterweights.
Here you see the setup with the STL-11000 and big filter wheel connected. 48 kg counterweight are barely enough for balancing the complete system.

As camera my STL-11000M was used together with the large 8-pos filter wheel.
The total weight of the OTA with the imaging train is at 106 lbs (48 kg)

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The CDK optics consists of three components - an ellipsoidal f/2.6 primary mirror, a 159 mm spherical secondary mirror and a 90 mm lens group for flatfielding. The optical system in total supplies effective 17" aperture (430 mm) at f/6.8 and at a focal lenght of 2938 mm. The corrected field covers 50 mm.The net weight of the OTA is 94 lbs (42.5 kg).

The OTA is fixed to the mount by a very sturdy dovetail adapter. The mirror cover is perfectly sealing the mirror box to avoid dust and spider deposits on the main mirror.

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As camera my STL-11000M was used together with the large 8-pos filter wheel. The total weight of the OTA with the imaging train is at 106 lbs (48 kg)

The image scale for the STL-11k is 0.63 arc sec per pixel, the field covers 41.5' x 27.7'

There are 3 fans in the back plate that provide an efficient cooling. The built-in temperature sensors for the mirror and ambient were showing values not more than 1 degree apart after 45 min of active cooling

The very stable 3.5" motorized focuser with encoders is handling the heavy CCD equipment without any problems

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Here you see the view from the secondary focus into the OTA. The light ring around the black baffling tube is indicating that the secondary mirror is oversized compared to the diameter of the baffling. This helps to reduce effectively possible reflections from stars close to the border of the field.

The illumination of the field is very even, checkout a typical flat: here.

The spider is very stable, fine collimation was done easily and holds up very well when slewing to different areas of the sky.

Improvements can be done at the holder of the secondary that is equipped with just 4 push screws. A design as for the CDK 20 using 3 push-pull screws would be preferable.

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A star test was performed, however at bad seeing. Here you find a video showing a bright focused star at f/20 with a b/w webcam.

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Here you find a video showing a sequence from extrafocal to intrafocal position at f/20 with a b/w webcam.

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The autoguiding was showing rms deviations of typical 0.4-0.5 arc sec, this was caused by the home made light weight pier that was not as stable as it should be.

Also the GM2000 mount is at it's limit with the 50 kg net load. The features of the mount control are impressive like pier flip, PEC, multi star pointing modeling;
The recalibration is quite helpful (press enter for 3 sec) for exact repositioning a target.

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After all information - what counts is the imaging performance. The not too slow system with generous aperture is producing amazingly detailed images at rather short exposures. The stars are of pinpoint shape out to the corners of the 24x36 mm CCD field.

Here you see a straight RGB on the western Veil using in total 60 min (20 min for each RGB) exposure time
(new Baader LRGB filters).

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Here you see a straight LRGB on NGC206 within the Andromeda galaxy using in total 100 min exposure time (new Baader LRGB filters).
Same as the previous image this one is completely uncropped.

I could not see a significant focus drift for 2-3 h of imaging time at falling temperature.
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I am quite satisfied with the optical performance of this new product. The mechanical design and fabrication details are of excellent quality. It is a pity that I could not take full advantage of the capabilities of this instrument caused by the moderate seeing conditions during the rare clear nights of the test period.


More information at the manufacturer's site for the CDK17:
Clic for the German page: here.
Find information on the GM2000 mount: here.
Clic for the German mount page: here.


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