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This kit modifies the imaging train of the Planewave CDK telescopes (CDK 12.5 – CDK14 – CDK17) to enable the use of an Off-Axis Guider (OAG) in conjunction with the Planewave 0.66x focal reducer to achieve a fast focal ratio, a much wider field of view, and autoguiding capability.
Article on testing and performance found here.
Performance tests performed on the same night with native vs reduced configuration.
The Soul Nebula was was imaged for 10x600seconds at a longer native focal length with a very fine image scale and then the system was switched to the shorter focal length configuration and the same target was imaged again for 10x600seconds (about 2 hours).
Moving from f/7.2 to f/4.75 increases the brightness over 200%.
Having the brighter images with the reducer allows using much less integration time. Below is a comparison 10x600s at native –vs– 6x600s with the reducer.
Both visual and Pixinsight SNR script verify the results.
Below is side by side of 10x600s at the native focal length, then swapped to the reducer setup on the same night. The object will be smaller but the here we unsampled 150% in Pixinsight and there was no loss in resolution even though the target was much lower by then.
It didn’t seem fair to compare the reducer images at a lower altitude than the native. Below is a comparison with the reducer images taken the next night at the same altitude of the native images. Also 10 x 600s. To match the image scale, the reducer image was unsampled 150% in Pixinsight. The image is actually sharper with smaller FWHM with the reducer.
This allows the user to swap back to the native focal length configuration in minutes. The procedure involves loosening three set screws, releasing the reducer from the OAG, and replacing the reducer with another custom machined part that maintains the original native focus position.
This component has a dovetail connection that simply plugs into the OAG with the camera on the other end. The focuser and OAG don’t need to be altered. Internal baffles ensure stray light and reflections are eliminated.
The majority of these CDK telescopes used sensors like the Kodak 16803 that had large 9 micron pixels and sensors. The resulting native image scale of 0.6~0.7 arcseconds/pixel was in the sweet spot for resolution and good signal to noise ratio buildup.
With today’s modern low noise CMOS cameras like the Sony IMX571 and IMX455, the smaller 3.76 micron pixels often lead to oversampling at a native image scale of about 0.3″/pixel. Oversampling comes at the cost of requiring a lot more integration time to build up high signal to noise ratio.
The resulting image is much brighter than the native one – about 240%. Image resolution is still very high at 0.37~0.46 arcseconds/pixel (depending on CDK model). Since the system will be oversampling in almost all cases, there is no loss in true resolution. A “larger” image of small targets can be produced simply by upsampling the already sharp reducer image.
The field of view (FOV) is limited given the smaller sensors. The dedicated Planewave focal reducer allows smaller sensors to capture a much larger field of view, thus reducing the overall cost associated with large cameras, filters, and filter wheels.
Incorporating an OAG into the system means no more guiding and tracking errors regardless of the sub exposure length, type of mount, lack of encoders, or perfect polar alignment.
The modification calls for several custom CNC machined adapters that position all the optical and mechanical elements at very specific positions.
Extensive testing has been done to compile charts and graphs that allow calculating adapters and spacing that are unique to each Telescope – Focuser – Camera combination.
Each system can be designed with custom components and to fit the users requirements.
Tolerances need to be to accurate to the millimeter from the focal reducer’s output flange to the camera sensor. This can be fine tuned by 0.1mm by user.
Distance from the telescope backplate to the input of the reducer is also critical, which in turn varies as the camera spacing is altered.
Furthermore, provisions are needed to position the guide camera at a specific position which is dependent on the other spacing values above.
Guide camera adapters and components are provided with the kit. Some suitable guide cameras are the ZWO ASI174mm or the ASI432MM tested with this kit here.
Each CDK model has specific spacing requirements, one design does not fit all. Custom adapters are designed, CNC machined, anodized, with internal surfaces treated with super anti-reflective acrylic.
Design and consultation is project dependent:
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