The Balancing Act

To use the "full cage" design, it seemed important to keep the weight near the poles to avoid torsion that might introduce collimation error.

Originally, I designed with the idea of balancing the weight at the center of the poles, but changed my mind when I remembered that most unstressed (near perfect balance) systems usually vibrate -- I didn't want that either.

So I settled for a design that would place the weight at the inside edge of the tube holes.

As you can see, I missed by about 1/4-inch, but no harm done as it turns out. Works fine!

The upper cage (including all components and an eyepiece) comes in just under eight pounds. As best I could determine, the imbalance at the outer edge adds a torque of about 1/2 foot-lb, which has caused no collimation error at all. We measured that by putting in a laser collimator and moving the tube through the entire azimuth cycle, then star testing at different positions. No error appeared.

Though it's not obvious, the liner is "inside out." Usually the granular side is placed inside, with the smooth side out. I wanted to try to reverse this for two reasons: first, I wanted to see how well the ABS would take Krylon Ultraflat Black. Answer: very well. Also, I wanted a smooth surface to eventually put some of Bryan Greer's flocked paper on (not yet done).

Both from Protostar. This is, I understand, the largest three-vane spider Bryan ever sold. It's great!