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Old 19-01-2024, 08:59 AM
Stefan Buda
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16" Dall-Kirkham rebuild

This instrument has been my planetary workhorse for over 20 years and is in desperate need of refurbishment and upgrade. I've been wanting to do this for several years now and finally seem to have the time to get stuck into it.

As there are only three types of telescopes, in my opinion, that are good for cutting edge planetary imaging, and one of them being the Dall-Kirkham, I thought there might be some people on the forum that could be interested in learning about this type of telescope. I'll be happy to answer any related questions.

My intension is to convert the Alt-Az mount to Equatorial fork mount and get rid of the field rotator mechanism.
A new electronic focuser will need to be designed and made.
I intend to simplify the primary flotation system.
The active cooling system will be retained as it proved itself to be useful.
The old Bartels drive will be replaced by OnStep controller.
All carbon fibre surfaces need sanding and coating with UV resistant polyurethane.
I may even regrind the mirrors because back in 2000 when I made them I did not have an interferometer and the figuring was done with Coude Mask and Ross Null testing.
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  #2  
Old 19-01-2024, 09:24 AM
Rod
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This is exciting! Iíve always wanted to build a cassegrain. What Focal ratio is yours?

Which are the other 2 planetary scopes you like? Iím guessing a long focus Newtonian and a Schiespiegler?

Rod.
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  #3  
Old 19-01-2024, 09:38 AM
Stefan Buda
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Hi Rod,

A Dall-Kirkham shouldn't be made faster than f/15 because of the excessive coma. Mine is f/16 so the focal length is 6500mm.

For planetary imaging aperture rules and I consider 10" as entry level but 12" gets you close to the cutting edge. If you accept that, then you are left with the proven C14s, the large Newtonians and the large D-Ks. The D-K being more of a DIY option as I don't know of any available commercially.
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Old 23-01-2024, 12:15 PM
sfarndell (Scott)
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Following with interest Stefan. A DK is next level challenging compared to a Newt as you also have to figure the secondary.
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  #5  
Old 23-01-2024, 02:49 PM
Stefan Buda
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Hi Scott,

I don't think a DK is more difficult to make than a Newt, apart from having to perforate the primary. I suppose what makes the Newt easier is that people buy the secondary off the shelf.
Keep in mind that the DK secondary is spherical and can be tested (figured) against a small test spere that is easily made.
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Old 24-01-2024, 10:32 AM
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What an incredible instrument! Having read your previous thread (yes... I went through and read all the pages) about the CDK builds, I know that this scope is going to be an absolute weapon once you're done the refirb!

Can't wait to watch your progress on this - and to see the final result!
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Old 25-01-2024, 08:22 PM
Stefan Buda
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Originally Posted by AlexN View Post
What an incredible instrument! Having read your previous thread (yes... I went through and read all the pages) about the CDK builds, I know that this scope is going to be an absolute weapon once you're done the refirb!

Can't wait to watch your progress on this - and to see the final result!
Thanks Alex for the encouragement and it's nice to know that you enjoyed going through my "phlog".

Meanwhile the new focuser parts are getting made and in a couple of days I should be able to upload some photos of the assembly.
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Old 26-01-2024, 06:26 PM
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Rainmaker (Matt)
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Subscribed !!! Your workmanship is music for my eyes !
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  #9  
Old 26-01-2024, 09:55 PM
sharpiel
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I wish you had a YouTube channel we could watch you work.
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  #10  
Old 27-01-2024, 07:50 AM
croweater (Richard)
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Quote:
Originally Posted by Stefan Buda View Post
Thanks Alex for the encouragement and it's nice to know that you enjoyed going through my "phlog".

Meanwhile the new focuser parts are getting made and in a couple of days I should be able to upload some photos of the assembly.
Love seeing what some of our skilled members are capable of. Looking forward to your next photos Stefan.
Cheers, Richard
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Old 27-01-2024, 03:23 PM
Stefan Buda
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Thanks, guys, for the feedback. Good to know that I'm not wasting my time uploading the images.

Well, the new back end is finished. I could have used a CDK250 focuser but I wanted a longer stroke in order to accommodate various configurations without the need to make many different adaptors.
So basically I modified the CDK focuser design by providing it with an extra long inner sleeve and a longer rack. I also decided to use adjustable teflon pads instead of ball bearings. I have designed many different focusers over the years but never one with teflon pads, and I wanted to see how good that option is. So far it seems promising - it was quite easy to adjust for concentricity, on the lathe, and now it is just a matter of how well it will take the test of time.
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Old 29-01-2024, 09:29 AM
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Very nice focuser Stephan, what do you think of the Teflon pads vs bearings?
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  #13  
Old 29-01-2024, 09:00 PM
Stefan Buda
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Thanks Joshua, my first impression is good. Of course, there is more friction than with ball bearings but not too bad, although this extra friction is expected to add a bit of hysteresis to the existing backlash in the gearing of the EAF.
Always focusing inward should take care of that.
Also remains to be seen how it will be affected by the large temperature swings on the long run - it gets very hot under the tarp in summer.
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Old 30-01-2024, 02:33 PM
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A little bit more progress: I 3D printed new ABS grills for the ventilation fans, to replace the old rusty metal ones. The fans are also getting 3D printed skirts to reduce spider habitat and to improve esthetics.
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Old 31-01-2024, 08:44 PM
Stefan Buda
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Time for astro strip tease: The private parts of a dedicated planetary telescope are getting revealed.
Such as the very sexy orange Byers worm gear.
By the way, the whole truss structure with the secondary cage, as seen on the third picture, weighs only 2.6kg, and the big tube only 1kg.
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  #16  
Old 01-02-2024, 09:02 AM
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Hi Stefan,

If this is a planetary scope, why would you bother with an equatorial set up or a de-rotator? I would have thought that both the exposure times being so short are not a problem and the imaging software takes care of any rotation in the stacking and alignment process. I have no such issues with this and I only use an altaz mount for lunar and planetary imaging. If things can be kept simple and technology gives you an advantage this way, why then go with an eq rig? Or am I missing something?

Alex.
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Old 01-02-2024, 09:31 AM
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Quote:
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I would have thought that both the exposure times being so short are not a problem and the imaging software takes care of any rotation in the stacking and alignment process.
Thatís only for the nearby planets, not the Kepler lot
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Old 01-02-2024, 01:55 PM
Stefan Buda
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Originally Posted by mental4astro View Post
Hi Stefan,

If this is a planetary scope, why would you bother with an equatorial set up or a de-rotator? I would have thought that both the exposure times being so short are not a problem and the imaging software takes care of any rotation in the stacking and alignment process. I have no such issues with this and I only use an altaz mount for lunar and planetary imaging. If things can be kept simple and technology gives you an advantage this way, why then go with an eq rig? Or am I missing something?

Alex.
Alex you are correct regarding the lack of need of a derotator for planetary imaging. Software can take care of that but when I designed this scope that was not the case.
When I think about the simplicity of a system, I also include the software that makes it work. For an off the shelf system who cares what's under the bonnet, but this is a large DIY instrument and I have to rely on open source solutions to drive it. The old Bartels drive is very obsolete and an EQ mount can be driven even by a simple Arduino clock drive if no GOTO is required.
Another thing I didn't like about the Alt-Az was the fact that during lunar imaging it was difficult to know which way were the cardinal directions.
With an EQ mount I can have the camera lined up and it will not change orientation.
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Old 01-02-2024, 04:12 PM
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It would be interesting to know how the movement of the draw tube feels while racking in and out, in terms of resistance, while having enough force to prevent slop but not to much that it binds on the pads.


Is the primary mirror box made from CF composite?
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Old 01-02-2024, 10:06 PM
Stefan Buda
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The movement feels smooth and without stickiness, but the friction is just whatever I think it should be acceptable. The adjustable plugs are M10 x 0.75 and they don't take much to overdo the tightness/friction.
Teflon's high thermal coeficient of expansion is a bit of a worry because I made the pads 3mm thick and I'm not sure what happens when the whole thing gets heated to something like 80 degrees Celsius. I might put it in the oven tomorrow and find out. If it gets sloppy after that, then I'll reduce the thickness of the pads to less than a millimeter.

The whole OTA is made of CF composite but it incorporates aluminium inserts in strategic locations. It also contains laser cut MDF in a few places.
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