Astronomy Hacks

Counterweight a Dobsonian Scope

Balance your Dob perfectly without using springs, clutches, or other kludges.

Traditional equatorial and alt-azimuth mounts lock the scope into position and drive it using gears. For small changes in direction, you move the scope by driving the gears with motors or manual slow motion controls. For large changes in direction, some mounts allow you to disengage the gears, pivot the scope, and then re-engage the gears. But while you are observing, the scope is always clamped into position.

Conversely, a Dobsonian mount uses simple friction bearings that allow the optical tube assembly (OTA) to move freely. When you want to point the OTA in a Dobsonian mount to a different part of the sky, you simply push the OTA up or down, right or left. The bearing friction must be small enough to allow smooth motions, but at the same time the friction of the altitude bearings—the bearings that support vertical movement—must be large enough to keep the OTA from shifting position under its own weight.

The altitude bearings must also have sufficient friction to accommodate small changes in weight. For example, removing one eyepiece before inserting another unbalances the scope because the front of the scope is suddenly lighter. The altitude bearings must have sufficient friction to keep the scope from swinging upward. Conversely, if you replace a light 1.25” eyepiece with a heavy 2” eyepiece, the front of the scope may have an extra pound or more pressing it down. If ...