Balancing a GEM

If the CAT came with a GEM mount, balance the tube before proceeding further. The manual should have instructions for doing that, but it is pretty simple. To balance in RA, undo the RA lock, holding onto the tube so it does not slam into the tripod if it is way out of balance, and move the scope until the counterweight bar and the scope are horizontal (Plate 35). Note which way the scope tends to sink when it is released, tighten the RA lock again, partially loosen the setscrew on a counterweight, and move it up or down the shaft as appropriate to balance in RA. If there is more than one counterweight, both may have to be moved. When the weights are in position, tighten their setscrews firmly, unlock the right ascension lock again, move scope and weights back to horizontal, let go while keeping a hand close to the telescope, and see if it is now in balance. If not, lock the RA lock and do some more counterweight moving until RA balance is achieved.

To balance the telescope in declination, the OTA will be moved forward or back in the mount's cradle. Exercise extreme caution while doing this. Only loosen the cradle bolts a small amount, just enough to allow the OTA to slide forward and back. Before loosening these bolts, check the telescope's current declination balance. Return the counterweight bar to the horizontal position and lock it down, undo the declination lock, and move the tube in declination until it is level as shown in Plate 35. Then, carefully let go of the tube and see if it moves. Does the front end sink, or does the back end sink? If neither, the scope is in good declination balance. If it does move, tighten the declination lock again, undo the RA lock, and position the

Plate 35. (Balancing a GEM) Good balance is important for accurate GEM go-to and tracking, but is easy to achieve. Credit: Author.

telescope with the counterweights down. If the scope was front-end heavy, loosen the cradle bolts and move the tube back a little. If the back end was heavy, move it forward. Tighten the bolts securely, bring the RA axis back to horizontal, tighten the RA lock, undo the declination lock, and allow the scope to move freely. If it moves, repeat the process until the scope is balanced in declination.

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