Counterweight

Hello,
I have a quick question for everyone........ How much counterweight have you needed for your Dobsonian mounted Newtonians to keep eyepieces, encoders, finders, etc. from tiping over?

Thanks much,
Jay

Jay
I needed about 10# to counterbalance a Denkmeier BV with eyepieces in a SM11" f/5.4.
Buck

Jay
I have been able to use my GSO 12" dob without counterweights. At the mirror end I have added Dob Knobs for collimation and a piece of masonite to hold the cooling fan. The 8AA fan battery pack is also mounted at the end. Altitude control is effected by Scope Grips from the Astronomy Shoppe. The greatest weight used is my barlowed 2" 40mm Paragon, 12X70 Russell finder and Telrad. There was no noticeable additional difficulty in controlling scope movement. Tension is added or subtracted using the Scope Grip knobs as needed. YMMV.

Hi Jay,
I use about 1.5lb on the 10" and 3lb on the 14.5".
The 10" only needs this for the heavy EPs, 31mm Nagler or the Binoviewer.
The 14.5" has a lot of stuff hung on the upper part that needs counterbalance or else it would work without counter weights for all but the heaviest loads and then only near the horizon.

Jay,

In a properly designed and executed Dob, there is no counterweight needed.

A Dob is supposed to be engineered such that the secondary (eyepeices and finders and all) and primary assemblies balance perfectly on the bearings.

However, that is harder to do than to say. And it gets really hard if you have a variety of things to add up there in the secondary. A simple plossl weighs a whole lot less than a binoviewer with two 31 Naglers. So--the real answer to your question is....it should take no counterweights with your standard eyepiece arrangement. But practically speaking, it may be easier to add weights as needed than to adjust the Dob in any other way when changing the setup or if it does not balance right in the first place.

(Before those with weight down on the primary box go off on me, I will concede that you can get a shorter scope some times by adding weight down below, but.....)

Alex

Thanks for all your replies. I have seen a variable counter weight system for truss dobsonians. Do they still make it?

Jay

After adding a ring bearing to make a rotating eyepiece section, PLUS a binoviewer, PLUS a lead-acid battery on the opposite side from the binoviewer to balance it on the ring bearing, I needed a LOT of counterweight on the bottom end of my 14" scope (about 15 lbs). At the bottom end I added a cooling fan/battery assembly which also has some 5/8" bolts, nuts, and washers for precise balancing. I split the Sonotube into two pieces but even at that it's near the limit of what I can lift.

On my new 20" truss scope, I planned from the beginning to use a removable fan/battery/counterweight assembly behind the mirror. The mirror box is a few inches too short for proper balance by itself, but the fan/battery/counterweight balances it perfectly.

When not using the binoviewer, I substitute a light shield assembly that hangs on the eyepiece drawtube making the telescope balance the same as if the binoviewer were in place.

I have also experimented with a "clothesline counterweight" made from a loop of cord, a pair of pulleys, a pair of S hooks that hook on either end of the tube, a spring, a turnbuckle to adjust tension, and a small weight.

It's also possible to balance the scope using springs, but you have to design it carefully so the springs exert the right amount of force at all angles.