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Old 04-04-2018, 06:02 PM
Stefan Buda
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Stefan Buda is offline
 
Join Date: Jul 2013
Location: Melbourne, VIC
Posts: 823
Ok, plenty of interest, it seems.

First a bit about the "why", before going on with the "how".
As decent size astrographs go, that have enough focal length to resolve smaller targets, there are not many practical choices, especially if the instrument needs to be portable. Three main types come to mind: The large Newt with Wynne corrector; The RC with field flattener, and the CDK. Of course there are other, more exotic, designs that I did not consider to be really practical, for various reasons.
Although the old Newtonian with a good corrector is theoretically a wonderful astrograph, in practice there is no nice and proper way of getting the focal plane perpendicular to the axis of the focuser. And that kills it for me, and besides, it is such a 20th century thing - like a DSLR.
The RC with well made optics and mechanics is also excellent, but that is where the problems are buried, because making those highly aspheric surfaces and keeping them aligned is not easy. To make those difficulties manageable, manufacturers keep the f/ratio rather slow.
Then there is the CDK with easy to make secondary and not too highly aspheric primary that can be made very compact and easy to collimate.
So, for me the CDK is the winner.

Some of the specs I'm aiming for:

-Carbon sandwich tube about 500mm long and 300mm OD.
-Should be twice as fast as an RC.
-Only secondary mirror collimation.
-Secondary mirror focusing, no focuser proper.
-Well corrected field of 44mm diameter.
-ASCOM interface for controlling focus, fans and heaters.
-Minimalistic appearance-no "Bob's knobs" anywhere.
-Should eat larger scopes for breakfast.

The mandrel for making the tube is coming along nicely. About 290 hours of 3D printing went into it so far and another 42 to go.
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