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How can I improve the UC18" Obsession?

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#1 Mike Eddigton

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Posted 26 July 2021 - 10:09 AM

Hi all,

I am a big fan of Dave Kriege's Obsession Classics Telescopes and with the help of his wonderfull book "The Dobsonian Telescope" I build and grind 12",18" 24" Obsession Classics style telescopes.

Now I bought a used UC18 with good optics, which magically fits in my little BMW Roadster, but the mechanical performance needs to be improved.

I think the ring on the upper side is to weak and the teflon bearings in altitude produce a jerky motion.
Can you give me some advice, how to improve the UC18? I want the mechanical performance of the Classics without loosing the compactness.

Clear Skies
Mike

#2 astrokeith

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Posted 26 July 2021 - 10:33 AM

Hi Mike

 

I have been 'reworking' a few UC's over the last couple of years.

 

The stability of the base board assembly is the first place I look. I think it is too weak, especially if the VMB is stored mounted on it complete with mirror. This leads to varying degrees of distortion of the base assembly. Almost any distortion at all leads to the sides (altitude bearings) not being absolutely vertical. This in turn leads to the trunnion bearings 'skating' on the high spots of the full length teflon bearing surface.

 

This produces a poor altitude movement, and also allows the VMB to slide and rotate on the base, rather than rotate via the proper centre pivot.

 

So check the sides first. I mark the entire surface of the full length teflon bearing strips with a sharpie, then put the scope back on and run it up and down etc, remove the scope and see where the sharpie hasn't been rubbed off. It its bad then the sides may need 'remedial action'!

 

The full length teflon bearings are a consequence of the folding trunnions (to avoid a bump as the hinge passes over), and is the weakest part of the design. The scope will always prefer to 'skate' on it rather than be firmly supported at 4 points like a classic 4 teflon pad layout. A good partial fix I have found is to replace the 4 white round furniture feet that stop the scope sliding too far sideways, with something both more substantial and that takes out the side to side slack. Usually there is about 3-6mm of side ways movement which needs to be removed.

 

The top ring is a bit weak, especially when combined with only 6 poles. I've found it tolerable though, especially if the scope is driven.  It dosent help that the pole end brackets are undersized and aren't axial with the poles. Pushing the top ring thus tends to bend the poles.

 

There are other issues, but I've already been gloomy enough! You're right in that the optics will be great. The UC concept is excellent but the Obsession version is a compromise. I built my own (I have to get it into a Mustang) but made my trunnions removable.


Edited by astrokeith, 26 July 2021 - 11:22 AM.

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#3 Starman1

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Posted 27 July 2021 - 02:36 PM

Hi all,

I am a big fan of Dave Kriege's Obsession Classics Telescopes and with the help of his wonderfull book "The Dobsonian Telescope" I build and grind 12",18" 24" Obsession Classics style telescopes.

Now I bought a used UC18 with good optics, which magically fits in my little BMW Roadster, but the mechanical performance needs to be improved.

I think the ring on the upper side is to weak and the teflon bearings in altitude produce a jerky motion.
Can you give me some advice, how to improve the UC18? I want the mechanical performance of the Classics without loosing the compactness.

Clear Skies
Mike

Double or triple the thickness of the top ring.

Increase the pole diameter to 1.5".

Use soap (dry bar soap) on the bearings to make the motion smoother.

Add counterweights to the bottom to compensate for top changes.

Add braces between the poles (to form an "A").

Add buttresses to the focuser plate or support the focuser with braces from top of focuser plate to top ring on both sides.

Add an SCT-like dew shield to the top that sticks up a foot above the top ring all the way around.

Always use a full shroud.

Adjust the side hinges so the two trunnion halves are perfectly in line when opened (to avoid the "bump").

Add fans, especially side fans blowing across the mirror, and let them run for a couple hours before observing.

Replace seatbelt sling with Glatter wire sling at center of gravity.

Those are some improvements I can think of off the top of my head.


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#4 Mike Eddigton

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Posted 28 July 2021 - 12:18 AM

Thank you Astrokeith and Starman1!

CS Mike

#5 a__l

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Posted 28 July 2021 - 01:05 AM

I would have done it differently. Which has become a little less compact, but retained the weight.
An approximate path in this thread https://www.cloudyni...vy-uta-on-dobs/

This will require a complete overhaul of the UTA and some instrument. Carbon rings for 18" can be bought in –°hina.

The second point is the side stops made of steel and milled Teflon (along the clearances of your side slide).
The third is Teflon on rocker, here you have to think. This requires more information.

Fourth. Here's already recommended by Don. If you have a belt, you need to install a steel cable. See how Obsession did on his latest UCs.

 

Ps. I think that here (photo) you need to make a smooth joint from the laminate and install 4 Teflon pads on the rocker. Sorry, I have not found a larger photo size.

Attached Thumbnails

  • 18UC-VMB-200px.jpg

Edited by a__l, 28 July 2021 - 01:50 AM.


#6 a__l

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Posted 28 July 2021 - 05:20 AM

Something similar to this. But I have to hold in my hands your alt bearings, see how accurately they are made.

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  • bearing_p (1).jpg



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