Last updated: 16 February 2003
Sent: Saturday, February 15, 2003 18:14:16 From: firstname.lastname@example.org (paul.lenartowicz) Thanks for a useful and informative site - it was invaluable when I was deciding what telescope to but, and then helping to get the best out of it. In return I have a contribution. Keep up the excellent work (but not at the expense of star-gazing!) Paul Lenartowicz (United Kingdom)
Telescope balancing - ETX 125, but probably applicable to all models, subject to adjustment of weight size.
I have read about the need to keep the ETX balanced in order to minimise load/wear on the motors and gears, which seems eminently sensible advice. When I bought my ETX125 complete with dewshield I was immediately struck by the unbalancing effect of the latter - and as for adding a camera at the other end, its hard to imagine that the drivetrain will last long.
But how to do it? I started by designing something based on an aluminium 'U' channel (for rigidity) attached to the twin threaded holes on the underside of the tube, to which I intended to mount weights made adjustable by means of slots and win-nuts, but as I was playing about trying to work out the size of weights required I hit upon a very much simpler system, as follows:
1) To balance excess front weight (e.g. dewshield):
There is a small 'ledge' at the rear of the eyepiece mounting tube. A weight can be held lightly on this ledge, and secured by a strap around the eyepiece mounting tube itself: I found that a Velcro strap of the type where one side is hook and the other loop, the end passed through a plastic ring and pulled back on itself to provide tension. They are available in various lengths (and can be trimmed shorter), and naturally have a slight elasticity enable firm securing. A bit of felt or thin rubber glued to the weight avoids scratching the telescope while helping to prevent slippage. Because the weight is not on the tube axis, the amount of weight depends somewhat on the degree of elevation. I found three weights to give more than adequate range of balancing for the Meade plastic dewshield: 500 grams for horizontal to anything up to about 45°, and 250 grams for anything from around 45° to vertical. A weight around halfway between gives good balance between about 30° and 60° elevation, etc, allowing tailoring to convenience.
2) To balance excess rear weight:
The same type of mounting strap, but longer, can be used to attach a weight to the top of the telescope tube itself (or underside if preferred) at any position until balance is achieved. Again, felt or, better, rubber stuck to the weight will protect the telescope tube and prevent slipping. I have used the bubble-wrap originally supplied with the dewshield as a wrapping for the telescope tube so that when reversed for packing away the dewshield sits comfortably - that also seems to provide good protection and grip with a weight added. I think that to minimise loading on the drivetrain the less weight added overall is best, so go for a lighter weight nearer the front of the tube, rather than a heavier one further back. For my 35mm camera the 500gram weight already used at other times to balance the dewshield does perfectly.
As for the weights themselves, almost anything can be used, but to minimise bulk and thereby make attachment easier a dense material such as lead is best. I made my own from old lead pipes, making a rectangular block about 55mm by 21 mm in section (using the aforementioned aluminium channel as a mould), cutting into 20, 30 and 40 mm lengths top give me weights of about 250, 375 and 500 grams, painted to seal the lead surface.
The sketch may help visualise.
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