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The Moment Arm and You

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#1 mark8888

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Posted 25 April 2013 - 01:56 AM

Well, me specifically, but also you, perhaps. :grin:

So I have the Berlebach Planet tripod, and the DM6 mount with the 8" extension. At some point I will attempt to mount a 7" refractor on it and see how it goes. (If it doesnt work I'll go for the Losmandy HD tripod).

I know one possibility is that I can drill holes in the tripod, raise the spreader a couple of inches, and use the 50cm tray (or continue using the 37cm one in the new, higher position). This has been discussed in a recent thread. It would widen the legs and thus apparently help with the "moment arm" effect of using a relatively large refractor with the Planet.

I came up with another possible solution that I want to propose here and see if anyone thinks it might work, or is a bad idea for some reason. Instead of moving the spreader and widening the legs, I could put a 10 pound weight right above each foot of the tripod. This makes the tripod much less top-heavy, and much less "tippy". I have one such weight and have experimented with it above one of the feet (see below), it's very securely attached with no chance of slippage and it seems very promising. I also think the damping time will be extremely low. With two more weights on the other feet, it will make the tripod so heavy and stable that it will become kinda pier-like, I think.

What I don't know, though, is if this solution might eliminate the moment arm effect. I'm no physicist. Of course I can try it myself when the time comes, and see how stable it appears. But still, I wonder if it's a bad idea for some reason and if I really should be widening the legs. (Besides not trusting myself to do a solid spreader re-installation job, I also don't have much space and so the Planet in its normal, unwidened state is good for me... unless I really need to be widening the legs and thats the end of the story!) I suppose what I'm asking is, if it turns out that it seems very stable to me with my proposed solution, then is it stable? Or am I missing something about the reality of "moment arm" and the degree of spread of the legs of a tripod, regardless of how heavy it is and stable it seems? Thanks for any thoughts.....

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#2 JMW

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Posted 25 April 2013 - 05:24 AM

You will certainly lower the center of mass of the tripod with the 30 extra pounds a few inches off the ground. The lower mass will allow you to have a more stable setup with the legs at a minimal height. Keeping the spreader in the factory position puts the tripod top higher than it would be if you spread the legs wider. I notice you have the double leg clamps also which can't hurt. How much does does your 7 inch refractor weigh and how long is the scope?

Your working height of the tripod legs would depend on the length of the scope and how high you want the eyepiece off the ground when looking near the zenith. I have a Losmandy 12 inch extension plus the added thickness of the Losmandy to DM6 adapter at the top and the Losmandy adapter that is part of the Planet. The Losmandy extension works out to be 14 inches verses the DM6 8 inch extension. I am using a TEC 140. When doing outreach to young children I keep the legs in their shortest position. For myself I like the height of the DM6 mount saddle to be about 8 inches below the height of my eyeballs so the legs are extended significantly.

#3 EFT

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Posted 25 April 2013 - 10:36 AM

I certainly don't see anything wrong with what you are doing. A very heavy tripod will definitely be more stable. Other people with different tripods have done similar things by placing a battery on the spreader (different tripod of course), hanging a weight from the center post of the tripod and other methods.

Consider also the ground that you are setting up on. You are probably better off on a stable hard surface than on grass or dirt.

#4 mark8888

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Posted 25 April 2013 - 10:48 AM

Cool. Thanks guys. I pretty much just wanted to make sure no one would look at the pic above and say STOP NO DONT DO IT!!! I definitely do love the Planet and want to try to make it work.

How much does does your 7 inch refractor weigh and how long is the scope?


I used it with a TEC140 for a long time, and it was great... the next scope will be a TEC180, it appears the tube length is 45", the weight is 36.5 lbs.

So... I will get a couple more of those weights (which really sit perfectly on the little footrests by the feet of the Planet tripod, with a belt each to keep them snug) and try it out once I get everything assembled.

Consider also the ground that you are setting up on. You are probably better off on a stable hard surface than on grass or dirt.


Thanks I'll keep that in mind. I had planned to use it almost exclusively on a stable hard surface (other than the very frequent earthquakes but that's another matter) so that's good to know.






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