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#1
05-08-2021, 10:12 PM
 Stonius (Markus) Registered User Join Date: Mar 2015 Location: Melbourne Posts: 1,301
Formula for counterweights?

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
#2
05-08-2021, 11:36 PM
 raymo Registered User Join Date: Sep 2011 Location: margaret river, western australia Posts: 5,779
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

Last edited by raymo; 05-08-2021 at 11:43 PM. Reason: more text
#3
06-08-2021, 05:03 AM
 Top_oz (Tim) Registered User Join Date: Dec 2020 Location: Newcastle Posts: 21
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.

Last edited by Top_oz; 06-08-2021 at 05:14 AM.
#4
06-08-2021, 05:52 AM
 mura_gadi (Steve) SpeakingB4Thinking Join Date: Apr 2019 Location: Canberra Posts: 423
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

Last edited by mura_gadi; 06-08-2021 at 06:12 AM.
#5
06-08-2021, 07:15 AM
 Stonius (Markus) Registered User Join Date: Mar 2015 Location: Melbourne Posts: 1,301
Ah, thank you so much guys! That makes total sense!

Markus
#6
06-08-2021, 07:18 AM
 Rainmaker (Matt) Strictly Visual...... Join Date: Jun 2019 Location: Canberra Posts: 422
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
#7
06-08-2021, 09:16 AM
 RB (Andrew) Moderator Join Date: Aug 2005 Posts: 23,781
Quote:
 Originally Posted by Stonius 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
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
#8
06-08-2021, 10:12 AM
 Stonius (Markus) Registered User Join Date: Mar 2015 Location: Melbourne Posts: 1,301
Quote:
 Originally Posted by 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

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. ;-)
#9
06-08-2021, 12:03 PM
 RB (Andrew) Moderator Join Date: Aug 2005 Posts: 23,781
Quote:
 Originally Posted by 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. ;-)
That's the one !

#10
13-10-2021, 03:19 PM
 Reilly1502 (Reilly) Registered User Join Date: Apr 2021 Location: Perth, Australia Posts: 5
I'm actually glad I stumbled upon this thread, I was just guessing but I can actually rest assured now I was relatively close
#11
13-10-2021, 04:51 PM
 astro744 Registered User Join Date: Apr 2007 Posts: 1,080

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.

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