Making Telescope Counterweights

My current astronomy and astrophotography setup is based around the Sky-Watcher Star Adventurer 2i, an equatorial tracking mount intended for cameras and small telescopes. Its stated payload capacity is 11lbs / 5kg. My setup, consisting of the optical tube (6.5lb/3kg), camera, additional optics accessories, mounting accessories, and some other small items, typically exceeds this limit. Prior to this project, the telescope was slightly off-balance, with the provided counterweight being insufficient even at the end of the rod. The motor has done a good job of handling the imbalance so far, but I can assume that continuing to exceed the payload capacity and failing to properly balance the mount would accelerate component wear.

As I cannot make my telescope or camera lighter, I considered 2 solutions for balancing the mount. One idea was to increase the length of the counterweight rod, using nearly the same total weight to exert a greater moment to match that of the telescope. After doing some research, extending the rod seemed like a difficult solution to implement. One end of the rod has M8-1.25 threads that interface with the mount, and the other has M6-1.0 threads used to trap a washer for weight retention. The counterweight rod is 1/2” in diameter and about 10” long. Unfortunately, the thread mismatch meant I could not simply buy a second rod and attach them in series. I also did not find longer replacement rods that were cost-effective or compatible online, so I shifted focus to the other idea: increasing the weight.

After some research, I decided that adding more weight is not necessarily a bad thing if it eliminates the imbalance experienced by the motor. In theory, the tracking motor can more easily accommodate a greater total downward force than a significant rotating imbalance. (As it turns out, several people online claim this works better than a longer rod anyway, because a longer moment arm would increase vibrations.) Another counterweight (which can only be purchased with another rod) costs about $30-35. I decided I could do better, so I found two scrap steel cylinders for free that had been cut into only slightly.

I could have gotten by with only one weight, but I decided to make two in case the other would be useful for another project. At the time, I was curious about the possibility of using the AZ3 mount as a polar wedge and knew that doing so would require an additional weight system.

The first step was to file and sand down some surface flaws, namely old welding spatter. Next, I marked the center of each cylinder and drilled out a 1/2” hole for the rod. The steel was fairly hard, so I used several bits of increasing diameter for an easier cut. My next consideration would be securing the weights in position along the rod.

The original Star Adventurer counterweight has a cross-drilled threaded hole with a ball-end clamping knob, which tightens against a smooth rod. In a perfect world, I would do this with the new weight, but I wanted to try an easier option first. I found a piece of 2” aluminum round stock in a scrap bin and decided to cross-drill that first. My goal was to use the smaller, softer aluminum piece and the existing smaller counterweight to trap the new weight in position.

I planned to cross-drill the piece on a drill press and thread the hole by hand, but a very kind machinist offered to throw it on his mill and show me some ways to line it up more accurately. The resulting piece was probably a bit cleaner than what I would have made using the drill press.

Putting everything together, there was a small amount of rotational freedom which is probably not a huge issue. I will keep an eye on this in case it causes any shifting or vibrations. Unfortunately, ball-end clamping knobs like the one for the original weight were difficult to find. I was worried that a clamping knob or bolt with a standard flat end would damage the rod over time, but I was able to find a bolt without sharp edges. In the future, I might 3D print a knob to put on the head of the bolt for ease of use.

I originally planned to powder coat the new weights to match the finish of the original and protect the parts from corrosion. I may still do this in the future, but for now I have been using the new components as-is. Over several months of use I have encountered no corrosion issues.