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The bane of a German equatorial mount (GEM) is the need to shift the position of the counterweight each time the mount performs the "meridian flip." Explanation: A GEM cannot track or slew very far past the Meridian in either direction. To get to the other side, it must slew all way around through north, until the telescope is on the opposite side of the mount.
It is beneficial to provide an "east-heavy" weight bias, so the right ascension drive worm is pushing against gravity to eliminate gear lash in the drive. The usual practice is to position the counterweight farther from the mount when the telescope is on the west side of the mount (pointing eastward), and the counterweight is on the east side. After the flip, when the telescope is on the east side of the mount (pointing westward), the counterweight is moved inward, closer to the mount, and the weight of the telescope provides the east-side bias.
Unfortunately, the Meridian flip and the need to reposition the counterweight often occur in the wee hours of the morning, when I'm likely to be sleeping as the automated imaging session progresses. To avoid having to get up in the middle of the night to adjust the counterweight, I added this constant-bias system to my G-11 mount. (Credit for this idea belongs to Rainer Ehlert, who first mentioned it on the Yahoo Losmandy_users list).
The concept is simple. Loop a string around the RA hub, and suspend a weight. With this weight removed, adjust the counterweight for a neutral balance (no east or west bias). Then attach the weight to provide the constant east-side bias with the telescope on both sides of the mount.
A thin Nylon string suspends a plastic bottle containing 7¾ pounds (3.5 kg) of lead birdshot (the water shown in the photo above proved to be not heavy enough, so was replaced with the heavier lead; scroll down for a closeup). I machined a small disk with a groove in it from Delrin to direct the string downward at the appropriate position. This disk is bolted to an aluminum support bracket.
This photo shows details of the Delrin disk, and how it's mounted. Two machine bolts hidden beneath the flat pier plate secure the aluminum support bracket to the pier plate.
Here we see how the Nylon string wraps around the RA hub and hooks to the string looped around the declination bearing housing.
The weight consists of a plastic bottle containing 7¾ pounds (3.5 kg) of lead birdshot. The Nylon string hooks to a steel ring attached through a hole in the bottle cap. The 7¾-pound weight is equivalent to one pound (0.45 kg) of additional weight at the end of the 20" (508 mm) counterweight shaft.
