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Old 04-04-2018, 02:09 PM
Stefan Buda
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Making a CDK astrograph

Since I finished the Honders, about a month ago, I've been working on a new project - one that is a bit more down to earth than the BRH.
It is going to be a 250mm aperture, f/5.8 CDK. I want to show the people from OO UK how it is done properly.
If there is interest here, I will do a photo blog (should it be called a phlog?) like I did with the Honders?
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Old 04-04-2018, 03:22 PM
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Ohh yeah, bring it on. Looking forward to it already Stefan.

Are you going with a carbon tube or truss system?
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Old 04-04-2018, 03:31 PM
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Atmos (Colin)
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That would be fascinating to have another phlog
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Old 04-04-2018, 03:35 PM
croweater (Richard)
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Quote:
Originally Posted by Stefan Buda View Post
If there is interest here, I will do a photo blog (should it be called a phlog?) like I did with the Honders?
Yes please Stefan

Last edited by croweater; 04-04-2018 at 03:36 PM. Reason: Spelling
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Old 04-04-2018, 03:51 PM
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Lognic04 (Logan)
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Awesome, looking forward to seeing the making of a new telescope!
If you don't mind me asking, how did you learn to make telescopes, like the mirrors and correctors/lenses?
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Old 04-04-2018, 03:51 PM
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Big thumbs up from me too
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Old 04-04-2018, 04:18 PM
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multiweb (Marc)
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+1. Bring it on.
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Old 04-04-2018, 06:02 PM
Stefan Buda
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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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Old 04-04-2018, 06:05 PM
Stefan Buda
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Logan,

In the age of the "Google University" there is no excuse for not knowing something.
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Old 04-04-2018, 07:26 PM
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Stefan please don't skip any details on the tube, I'm watching with interest.
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Old 04-04-2018, 07:40 PM
sharpiel (Les)
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I'd like you to make me one too please...
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Old 04-04-2018, 07:40 PM
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Logan,

In the age of the "Google University" there is no excuse for not knowing something.
True
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Old 05-04-2018, 05:48 AM
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Definitelly interested in phlog
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  #14  
Old 05-04-2018, 06:24 AM
rally
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Hi Stefan

Great project, how are you making the corrector lens assembly ?

Rally
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Old 05-04-2018, 09:01 AM
Stefan Buda
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Quote:
Originally Posted by sharpiel View Post
I'd like you to make me one too please...
I am keeping tabs on labour and material costs and I will be able to tell you how much you would need to fork out.

Quote:
Originally Posted by rally View Post
Hi Stefan

Great project, how are you making the corrector lens assembly ?

Rally
Same as I did for the Honders: I cut up a few slabs of optical glass and turn the pieces into lenses using normal lens making techniques - see some pics on my earlier phlog: http://www.iceinspace.com.au/forum/s...=151040&page=2

In fact I tried to cut some corners by producing an optical design that used an off the shelf Edmunds PCX lens, but I abandoned the idea because the Edmunds lens is available only for 50mm diameter, which is not enough.
They have a larger version of that particular lens, but it comes only with single layer AR coatings, and that would not satisfy the last point of the above listed requirements.
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Old 05-04-2018, 09:02 AM
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Awesome, watching with interest Stefan.

Cheers... Josh
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  #17  
Old 05-04-2018, 09:10 AM
Stefan Buda
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Quote:
Originally Posted by Paul Haese View Post
Ohh yeah, bring it on. Looking forward to it already Stefan.

Are you going with a carbon tube or truss system?
Paul,
I think for a ten incher a carbon tube makes more sense than a carbon truss.
Probably from 12" up is a different matter.
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  #18  
Old 05-04-2018, 02:24 PM
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That's a great project.

I look forward to your results.

One point I would make about CDK designs having used a 17 inch now for several years, it would be good to minimise the vignetting. I believe there are optical compromises between vignetting and F ratio etc but I think less vignetting would be a good thing. It would make it less dependent on good flats.

Greg.
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Old 05-04-2018, 02:48 PM
Stefan Buda
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Hi Greg,
Thanks for the observations regarding vignetting. If you have any other recommendations, please don't hold back.
My design, as it stands now, has no vignetting up to an APS-C sensor size, and less than 20% at the corner of a full size chip. That is supposed to be quite good.
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  #20  
Old 05-04-2018, 02:56 PM
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Would this type of scope(and this one in particular) be comparable to a Newtonian in image quality, ease of use and cost?
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