Balancing your Dobsonian
Submitted: Tuesday, 10th April 2007 by Dennis Greeve
The owners of a dob scope are well aware of the difficulty to balance it as the angle of altitude changes. Add a Telrad or a laser pen, a camera and the balance changes.
I came up with an idea, which works. It is neat and fool proof. All one needs is to cast two lead weights, flat bars in my case, 250mm long 50mm wide x 10 mm thick that have previously been calculated by trial and error to balance the scope at two altitude angles, mid and low, with and without added photographic gear. Each weight is contained in a cloth bag, which hooks onto the mirror box.
Bag number 1 has short loops whereas bag number 2 has long loops. The principal being that as the scope is lowered from the vertical, bag number 1 is picked up and balances the scope with or without the camera until lowered further when bag number 2 is lifted and balances the scope through to the lowest elevation.
A Perspex plate screwed to the rocker box guides the two bags back to rest. If found necessary for a finer balance adjustment, the required weight could be divide into three bags. I had thoughts of joining the second bag to the first, but the second is only hooked on when required.
The same principal could be used on the Chinese tube style dob scopes. The bags wouldn’t pack away but that’s no big deal!
The unusual shaped hooks were necessary to prevent the bag loops from lifting off when the scope is raised to vertical. The Perspex guide retains the weights as the scope is raised or lowered.
A point of further interest, the altitude and azimuth rotation is carried on self-made roller bearings. Teflon pads suffer from sticksion: that is, the movement is restricted until started. I’m in the process of perfecting simple drives for both the altitude and azimuth and found the Teflon pads were a pain. With the balance sorted out the telescope is so much easier to move on rollers. The azimuth has 3 equally spaced roller bearings on the perimeter of the turning circle and a sealed ball race provides the turning pivot. The altitude has 2 rollers each side replacing the 4 Teflon pads.