Formula for counterweights?

[Stonius]

I'd like to calculate how much weight I'll need to add the the back end of my Dob to counterweight my eyepieces to maintain balance.



Is there a formula for figuring out how much weight to add to one side of a see-saw with a length x and fulcrum at position y that will achieve balance?


Like, a see-saw 1m long, where the fulcrum is at, say 33.333cm and the weight on the long end is 1kg. Will it require 2kg on the shorter end to counterbalance it? 4kg? I've no idea what the mathematics are here. I'm sure some long-bearded greek guy figured it out 2,000 odd years ago, but I don't know his name or the name of the theory he invented.


Many Thanks


Markus

[raymo]

If the distance from the pivot point to your eyepiece is for example 1200mm,
and the counterweight[s] will be mounted for example 400mm the other side
of the pivot point, then the counterweight[s] will need to be 3x the weight of the eyepiece, because the eyepiece is 3x as far from the pivot point as the weight[s] are, and yes, simple answer to your question, 2kg. Put simply, however many times further the eyepiece is from the pivot point than the weight[s] are, is how many times heavier the weight[s] have to be.
raymo

[Top_oz]

The moments around the pivot point need to sum to zero.

A moment is the centre of mass x distance from pivot.

So 1kg x 1m clockwise - 0.5kg x 2m anticlockwise = 0 and would be static.

The sign you give to each direction is arbitrary but you must be consistent.

Where this will get tricky if you want to be super accurate is friction at the pivot(ignore it) and the COG of your various masses but you should be able to get into the ballpark fairly easily.

[mura_gadi]

Hello,

As suggested already, it is mass X distance from the pivot points. The main thing to remember is the rough ratio, 1kg on the focuser is 6.5kg on the base sort of thing. That actual ratio is dependent on a large range of factors based on your scope.

You'll end up needing a lot of separate weights if you work it at each optical setup weight. So think about variable weight setups like a sliding weight, chains and hooks on the base etc. You should be able to mark off chains links or slide points for each ep later to make setting up easier.


Steve

[Stonius]

Ah, thank you so much guys! That makes total sense!


Markus

[Rainmaker]

A x B divided by C = D

A is distance in cm from pivot to focuser centreline
B is extra weight of eyepiece
C is distance in cm from pivot to where you want to attach the counterweight
( in the opposite direction )
D is weight of counterweight

P.S. Assuming that your scope already balances with at least one of your eyepieces, then you need to work on the difference of the weight of that eyepiece and the heavier ones...... but realistically a scope should not be that sensitive to changes unless you're going from a small plossl to a N31T5

[RB]

Yes Markus (Stonius), it was my great, great, great grandfather, Andreas Studakopoulos, who figured it all out.
The name of the forumla in Greek roughly translates to-
Momentous Fulcrumanus Heavyweightous Simplicitiosus.

RB
:D

[Stonius]

Ah yes, I think I've heard of him. Didn't he invent the automatic field rotator one day over a spannakopita, 1,600 years before Gallileo invented the telescope? Impressive chap. Before his time, some might say. ;-) :P

[RB]

That's the one !

:thumbsup: :lol:

[Reilly1502]

I'm actually glad I stumbled upon this thread, I was just guessing but I can actually rest assured now I was relatively close :D

[astro744]

I use this https://www.meade.com/tube-balance-weight-system-1405-for-meade-14-sct-and-acf.html#additional

on my Lightbridge 16” in addition the the Farpoint weights. I found that when I add a Paracorr with heavy eyepieces I needed some sort of variable weight system. Works well with just one weight. I mounted it on top of the lower tube assembly and slightly on the opposit side of the eyepiece to offset any lateral out of balance that would put more weight on one altitude bearing than the other.

[gunsworth]

Another solution is to dispense with fixed counterweights and use adjustable counterweights.
Using a finder scope or big eyepieces on my 16” Lightbridge causes imbalance. Changing eyepieces from a 2” 26mm to a 13mm really upsets the balance.
My magnetic counterweight uses a lead acid battery for mass. The battery also provides power to the fan and dew heaters without messy cabling coming up from the baseboard.
Moving the counterweight up or down the lower part of the scope is a quick and easy way to achieve balance.
If anyone is interested, I can provide details on construction and pics. (But I do need help on how to post pics)

[gunsworth]

Using a finder scope or large eyepieces on my 16” Lightbridge caused big imbalance. The need for a counterweight is clear.

This magnetic counterweight incorporates a battery and solves a second problem:
The second problem with dobs is conveniently getting power to the dew heaters and primary mirror fan. This counterweight uses a 12V battery (for power and mass) that makes the wiring to the fan and the dew heater very simple. The battery can easily be removed for swapping out and charging.
Magnets are used to make it easy to reposition the counterweight, depending on what’s at the top of the scope. Simply slide the counterweight up or down the scope.

See pic Counterweights 002 fitted (you probably need only one counterweight for a 12” dob.)
Each counterweight consists of:
• a 12V battery sandwiched between
• two pieces of timber
• a couple of magnets (from a dead microwave) to hold it to the scope
• some “rubbery” drawer liner material (to stop the magnets scratching the scope and give grip)
• fasteners to hold it together (T-nuts, threaded rod and threaded knobs) and
• a handle for convenience

See pic counterweight disassembled.
Magnets: You can pick up a dead microwave on council clean-ups. All microwaves contain 2 strong donut shaped magnets. These magnets are in the magnetron which is encased in metal and is close to the chamber. (Not the transformer, the other large part.) To get the magnets out, remove the microwave’s cover. Prise open the magnetron’s metal case. Pliers may be best.

TIP #1: definitely avoid putting a screwdriver through your hand when prising!
TIP #2: When you get the magnets out, do not test how well they attract – they will drag themselves out of your hands and smash. The pieces can’t be glued together again.
After that construction is pretty straightforward. Araldite glued the magnets to the timber, and silicone sealer attached the draw liner to the magnets.
TIP #3: Don’t put the magnets on the ground, they will pick up stuff that scratches your scope.

Ideally the counterweight should be positioned in the same plane as the elevation axis so that it is independent of the scopes elevation. In practice, this wasn’t a problem - I do not use the “brake.”
Questions welcome, but response might not be quick.