Counterweight Calculations

To figure out how much counterweight is needed to balance a particular scope you can calculate the torque that the scope exerts. You will need to estimate where the center of gravity for your scope and attachments is located. Then measure from that point to the axis of RA rotation. Once you have the distance, multiply that times the weight of the scope and accessories.

The illustration above shows an imaginary SCT that weighs 30 lbs. with its center of gravity 14.5" from the RA axis of rotation.

14.5" x 30 lbs = 435 in-lbs. of torque.

Next figure where you want the center of gravity of the weights. Because of the nature of moment arm there is an advantage to having more weight closer to the axis rotation than less weight further out. The location in the illustration allows room for adjustment and for adding heavier eyepieces, cameras, etc. So, figuring the distance of 10" from RA rotation we would divide the torque of the scope by the distance to the weights. The result would be the total required weight.

435 in-lbs / 10" = 43.5 lbs.

Two 21.7 lb. weights would suit this situation. Or, you could use three 14.5 lb weights.

To figure the center of gravity of a set of weights you need the length of each weight.

AP3-GT1HD-22lb and L3-G11-23lb are 3" long.

AP2-GT1HD-14lb and L2-G11-15lb are 2" long.

AP1-GT1HD-7lb and L1-G11-7lb are 1" long.

Astro-Physics 900GTO (approximate measurements)
= 7"
B = 7"
C = 13.5

Losmandy G-11 (approximate measurements)
A = 6.25"
B = 6.5"
C = 14"

One of my saddles, a DC11 dovetail bar, and a radius block for mounting on a C11 all total about 1.7".Add to that the radius of the scope (if the center of gravity is in the center of the scope).

With a piggybacked scope you would add the inch-lbs of the guide scope to the inch-lbs. of the main scope. If the main OTA weighs 15 lbs. and the guidescope weighs 7 lbs. the calculation would go something like this:

(Main OTA 15 lbs. x 10" = 150 inch-lbs.) + (Guidescope 7 lbs. x 17" = 119 inch-lbs.) =
OTA total 269 inch-lbs.

Notice that the guidescope is less than 50% the weight of the main OTA yet it requires about 80% as much counterweight as the main OTA.

There are several combinations of weights that could be used in this example.

1. Two AP2-14lb weights centered about 9" from the axis of rotaition.

2. One AP3-22lb centered about 12" from the axis of rotaition.

3. One AP2-14lb and one AP1-7lb centered about 12" from the axis of rotaition.

Back to weights for:

Astro-Physics G-11

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