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Dobsonian smooth AZ: lazy Suzan + Teflon Brakes

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#1 Andy-di-Notte

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Posted 26 June 2009 - 03:43 PM

Perhaps a perfect compromise for a smooth AZ movement:
Lazy Susan + Adjustable Teflon brakes.
This is for Dobsonian mounts like the GSO or Zhumell.

Just wanted to share a DIY modification of the azimuth bearings in my Guan Sheng (GSO) 200C Dobsonian Altazimuth mount. I hope it might be of use to someone! ;)

The original "lazy Susan" roller bearings in some modern GSO Dobsonian telescope mounts produce a very smooth movement in the azimuth axis.
Some of us find the movement to be too smooth making it difficult to fix an object properly in eyepiece without overshooting. This system is also so sensitive it can easily be bumped out of position when changing eyepieces or even just by a gust of wind.

Initially I tried to just add pads of material, such as felt, to add some friction, but it was tricky to cut the pads to the right thickness for the desired effect and impossible to get the right amount of friction without introducing stiction, the annoying jerkyness of movement when trying to move the mount in AZ by very small amounts.
As many Dobsonian owners will confirm, stiction, or static friction, will turn a seemingly smooth AZ movement into a irritatingly jerky one at high magnifications (240x or more).

In a second attempt at improving the AZ movement I decided to get rid of the lazy susan altogether and use traditional Teflon bearings.
I also tried applying either silicon grease or teflon based spray to the pads to further reduce stiction.

That worked fine in many respects, certainly better than the lazy-Susan by itself, and many people will be satisfied with this time proven approach.
But it is still not perfect ...
Especially with high magnifications and/or in combination with looking at objects near the zenith, the Teflon-only system is a bit too sticky and stiction prone.

The final modification:

Thanks to a tip from someone at a star party I decided to modify the Lazy Susan based system by adding adjustable brakes.
After some testing it seems this brings the best of the Teflon and Lazy-Susan methods together into one system.

For the brakes I tried a few different materials from rougher to smoother, and in the end the material that gave the best results was the smoothest: Teflon.
With the Teflon brakes I could introduce enough friction to slow down the super smooth movement of the Lazy Suzan bearings and still have the lowest possible stiction. The advantage with adjustable brakes as apposed to
pads, is that you can adjust the friction very accurately and easily even while you're observing. You may want to alter the friction depending on the telescope orientation, magnification or just for a change! :-)

I've attached some photo's in case someone else might want to attempt this modification.
Here's a description:

Image 1):
Stuff needed for the braking system: wing-bolt and washer, teflon pad and T-nut + a hole has been drilled in the rocker box for the T-nut.
Posted Image


Image 2):
(1) The T-nut is inserted into the hole (underneath the rockerbox board).(2)The wing bolt is inserted into the T-nut. (3) The washer is glued to tip of the wing bolt with a gel type super glue. (4) Then finally the Teflon pad is stuck to the washer with double sided tape.
Posted Image


Image 3):
Final result, Dobsonian mount with Lazy-Susan + two adjustable Teflon brakes.
Posted Image

#2 Dhellis59

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Posted 12 June 2013 - 04:21 PM

Excellent idea. I was seeking something like this due to adding a lazy Susan to my xx14i. When using an EQ platform, the DOB would sometimes move due solely to the force of gravity. Thanks for sharing.

Darryl

#3 BSJ

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Posted 13 June 2013 - 07:22 AM

It looks like you removed the original center knob.

On mine, with the large knob that came with it, I can easily adjust the required tension...

#4 Matt Wallin

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Posted 17 June 2013 - 10:42 AM

This is a great idea, my GSO (Zhumell) dob has way too free of motion on the az axis. I crank down the center knob when it is windy or I'm on slightly uneven ground, but it takes so much tension I almost cringe to think about it. I am getting ready to build an eq platform, been pondering how I should address the forces induced by the tilting platform on this all-too-free spinning axis. Thanks for the ideas!






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