Balancing a GEM in all planes.

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Here the telescopes have overshot the parking position. The weights should be low and in line with the mounting. The telescope tubes should be horizontal.

While forcing a deliberate Meridian Flip the motors stalled with the OTAs near the vertical. So I loosened all 6 clutch screws and rebalanced the mounting and OTAs.

The considerable torque required to drive and resist unwanted movement of the OTAs demands stiff clutches. The telescopes must always be motor driven around the sky and never pushed independently of the motors. Otherwise the precise count of motor rotations would be lost. Along with the telescopes' precise pointing position on the sky.

The Beacon Hill wormwheel sets each had only one screw pushing a small plastic pad onto their respective axes shafts. This was found to be inadequate with such long and heavy telescopes. So I added two more screws and pads at 120° intervals around the wheel bosses. Loosening all the screws is enough to allow accurate balancing but is not perfectly free of friction. The bosses themselves add their own friction along with the large, flange mounted, self aligning, ball bearings supporting the shafts.

It took some time and effort to achieve balance in all positions. This was mainly due to the offset, 6" H-alpha telescope fixed to the side of the saddle via tube rings. I chose this position to avoid needing more counterweighting on the Declination shaft. As would have occured if I had mounted the 6" onto the 7" tube. The weight of the 6" would also have been added to the load on the 7" tube rings.

I have ringed the fixed, offset weights which counterbalance the 6" H-alpha telescope in the image above. The smaller, brass weights on the sliding rail fixed to the saddle are used to balance heavy items on the focuser end. The great distance from the focuser to the axes on larger telescopes can easily produce dramatic changes in balance. The solar prism and binoviewers add nearly a kilo each. A long and heavy telescope is not easy to control when it suddenly wants to swing nose down!

A single telescope would not suffer from major offset forces. [Except for a heavy finder, if any.] My own system needed considerable reduction in the moment [mass x distance from the pivot] of the offset counterweights. I hadn't been able to judge this when balancing with the telescope tubes horizontal. Fortunately I have a selection of small, barbell weights from an Aldi special offer in 1/2, 1.25 and 1.5kg. Or roughly 1, 2 and 3lbs.

This shift in offset balance had to be managed without the main counterweights becoming dominant. The critical position was with the OTAs vertical. Only in this position do the offset forces display themselves and allow adjustment.

It's no use just balancing the OTAs when they are horizontal. Such large and heavy OTAs have to be balanced in all possible positions. So that the OTA's have no bias in any direction. It is often possible to sense a desire for the OTAs to continue moving on in a particular direction even when they stop when released. It takes considerable patience to achieve full neutrality.

This exercise must, of course, include any heavy accessories. A slight eastern bias is considered desirable to take up any backlash in the drives. The wormwheel teeth can then remain in continuous contact instead of rocking between teeth. The usual expectation is that drives will push the telescopes gently but constantly uphill towards the west to "follow the sky." Any imbalance would inevitably throw such ideal drives completely awry!

Click on any image for an enlargement.

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