Author Topic: counterbalancing a truss dob?  (Read 1686 times)

John Pierce

  • WebHack
  • Administrator
  • Observer
  • *****
  • Posts: 308
    • View Profile
counterbalancing a truss dob?
« on: October 21, 2016, 12:16:57 AM »
anyone have any ideas for how I add weight to the bottom of this?



I need to take a closeup of the bottom, next time I have it setup I'll do that.

anyways, it balances fine now with a telrad and one or another of my Explore Scientific 1.25" eyepieces...  but if I add my 9x50 erect finder, its way nose heavy.   I suppose I could move the finder shoe down to the side of the base box, hah!

mccarthymark

  • Observer
  • Posts: 127
    • View Profile
    • observing blog
Re: counterbalancing a truss dob?
« Reply #1 on: October 21, 2016, 02:08:41 PM »
I use the Markless counterweight system (http://www.marklessastronomics.com/counterweight%20products.html) on the front and back of the box.  Once I received it I realized there could be a way to accomplish the same with parts bought at a hardware store (aluminum T-channel in which you can slide a bolt head, washers, wood plugs for the barbell weights, and threaded knobs).

You could also use a metal bar or a metal pipe filled with sand or BBs strapped to the mirror cell.
Mark

John Pierce

  • WebHack
  • Administrator
  • Observer
  • *****
  • Posts: 308
    • View Profile
Re: counterbalancing a truss dob?
« Reply #2 on: October 22, 2016, 11:16:40 AM »
so here's the bottom...


that eyebolt in the middle apparently was for hanging a weight, but I don't like that its attached to the mirror mounting plate, so I'm thinking I'll secure something between the two vertical square bars, right at the bottom near that cross plate.   a pint of water on the eyebolt wasn't enough weight (bungee cord experiment last night).   thinking I need between 3 and 5 lbs to offset my 9x50 finder

astropoyson@sbcglobal.net

  • Observer
  • Posts: 3
    • View Profile
Re: counterbalancing a truss dob?
« Reply #3 on: October 25, 2016, 08:47:13 PM »
John,
I solved a similar dobsonian balance problem by adding two types of weights: (1) a standard one pound weight-lifting lifting weight permanently and for refinement:  (2) large ocean fishing weights, the latter attached with velcro.  I had dreams of calibrating the weights such that a particular weight was for a particular eyepiece.  But that never happened, ergo it was overkill.  The main problem is losing the velcro-ed weights in the dark and leaving them behind.  Somewhere in the universe there is a huge pile of fishing weights.  You would surely want to fasten the added weight to the OTA and not to the mirror cell, but you knew that.
Robert Shelton

John Pierce

  • WebHack
  • Administrator
  • Observer
  • *****
  • Posts: 308
    • View Profile
Re: counterbalancing a truss dob?
« Reply #4 on: October 27, 2016, 04:51:20 PM »
I'm suspecting I'm gonna need more than a couple lbs due to the extreme delta in lever distances... the mirror box already weighs 75 lbs...   back of the mirror box to the alt pivot axis is like 18", while the eyepiece and finder are like 83" off the ground, the pivot is 24" off the ground, so the eyepiece+finder weight is like 83-24" = 59" vs that 18", so 1 lb of finder needs ... hmm, 3.3 lbs of weight?    my finder weighs 1.25 lbs, so ok, maybe 5 lbs right on the bottom of the mirror box is perfect.

next time we get good weather, I'll have to setup the scope and play with some weights.

John Pierce

  • WebHack
  • Administrator
  • Observer
  • *****
  • Posts: 308
    • View Profile
Re: counterbalancing a truss dob?
« Reply #5 on: November 06, 2016, 01:51:03 PM »
and indeed, my calculation of 5 lbs was spot on, scope was perfectly balanced friday night with a 5 lb dumbbell bungied to that loop.   So, today, I took some heavy gauge 'plumbers tape' (that steel strapping stuff with lotsa holes that is used to hang pipes) between those two vertical box section beams (which, btw, are 1x2"), folded it around said beam, ran a bolt with nylock nut and a couple washers on each side to clamp said folded piece to said box section, then securely tie-wrapped my dumbbell to said plumbers tape.  nice and secure and solid.   I forgot to take a picture before I put the scope away, so next time its setup, I'll get a snap...

Don Pensack

  • Observer
  • Posts: 3
    • View Profile
    • EyepiecesEtc.
Re: counterbalancing a truss dob?
« Reply #6 on: August 12, 2017, 01:09:12 PM »
For looks, add two small finger-shaped pins to the bottom edge of the top side of the mirror box.
Buy a few dumbbell weights of 2.5 and 5 lbs.  Add the weights to the pins as needed.
If you only need one weight and not above 30° altitude, a bungee cord between the bottom edge of the top of the mirror box
and the rocker box below it will suffice as a "virtual" couterweight.  Just make sure the length of the bungee is the right length
that it doesn't start getting stretched until the scope is getting low.  As the cord stretches, the equivalent counterweight increases,
which is ideal.  If your eyepieces vary in weight a lot, a second hook and second bungee cord may be needed.