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How to add a counterweight to a Unitron AZ 114 or 140 mount
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How to add a counterweight to a Unitron AZ 114 or 140 mount
The Unitron Alt-AZ mounts are wonderful pieces of engineering. They are rigid, smooth with slo motions and locks on both axes that are a delight to use.
They can carry heavy loads, only limited by the tension of the springs in their tangent arm assemblies. You provide the twisting action of the slo mo knobs which pushes against these springs, and when you wind it back, the spring provides the force to follow. This design is simple and elegant. It is also backlash free as the spring is always pushing back. The one glaring deficiency is that like any these upright AZ designs, they are horribly out of balance in anything other than the horizontal position. The heavier the scope, the worse this is, and in worst case scenario’s the springs may not have enough oomph to push the scope back to the horizontal position.
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Unitron realized this in their larger Alt-AZ mounts where they engineered in a threaded ‘boss’ for a CW shaft and CW. This helps offset the weight of the OTA as you move away from the horizontal into the vertical position. It’s never perfectly in balance because of the variety of vectors as you move through the ALT positions.
But this counterweight does help the cause in as much as it effectively relieves the spring of having to do all the work, and also helps prevent catastrophic crashes if you inadvertently loosen the ALT lock without supporting the OTA. (which is good practice on any mount BTW) |
So, this article is about how to provide this same utility with the lighter 114 and 140 mounts.
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My solution was to replace one of the OTA securing wingnuts with a beveled extension that screws on in place of the wingnut (1/4-20unc) and provides about 20* of angle to allow the CW to clear the tripod hub in the horizontal position. |
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I used some 5/16” (8mm) steel bar I had lying around in my bits bin for the CW shaft, and threaded one end 5/16-18 to thread into the beveled nut and the other end was drill/tapped #10-24unc to receive some Celestron CW’s I had lying around.
It could just have easily been the more traditional CW that you slide up and down the shaft to achieve balance – but I went with what I had on hand. |
With this easy addition, the scope is now ‘almost’ in balance through most of its motion, and the springs have an easier time returning the scope to the horizontal position. Not so much a factor with the featherweight Unitron 114 OTA (~3.5lbs) but if you use heavier OTA’s, it helps a lot. I had a Televue Pronto mounted recently (~7.5 lbs) and if you were viewing the zenith, the springs didn’t have enough chutzpa to return the scope to the horizontal. With the addition of this counterweight system, it had no problem.
And the best thing is that it requires no alteration of the mount and is fully and easily reversible if you choose. (Providing you remember where you put that loose ¼-20 wingnut….)
- Astrojensen likes this
2 Comments
It's a good idea. I had a 4 inch Unitron alt-az and an original Vernonscope 92mm which was the heavier one. It definitely would have benefited from a counterweight as the weight of the instrument compressed the spring inside the mount which I guess after decades was a bit soft.
It not only takes the 'heavy lifting' off the springs, but because they are not in full compression mode all the time trying to handle the load, the slo-mo knobs are a bit smoother and easier to turn.