Balancing a GEM


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How to 3D balance your German Equatorial Mount

The following is the process (provided with permission) used by Niall Saunders to balance his EQ8 mount.


First, fit the all accessories you might typically expect to have fitted -
but remove/unplug as many cables as possible (the ones looping onto the OTA

  1.  Start with the OTA horizontal (pointing N or S, it doesn't matter) and
    the counterweight bar 'horizontal' (true East or true West, it doesn't
  2.  Tighten the RA axis clutch
  3.  Loosen the Dec clutch - do this fully, and make sure the axis is
    really 'loose' (but be ready for any sudden 'fall-away' of the OTA)
  4.  Move the nose of the OTA over, very slightly - no more than a couple
    of inches or so - up or down (test both)
  5.  Does it stay there, return, or fall further away in the direction it
    was moved?
  6.  (a 'difficult step') Add weight to the centre-line of the OTA axis, or
    move the OTA dovetail in the EQ8 saddle-clamp until the OTA is balanced. I
    like to keep my dovetail 'centered' in the saddle-clamp - for maximum
    rigidity - and use an extra dovetail bar, mounted cross-ways to the main
    OTA dovetail. I then mount weight(s) to this dovetail to give a fully
    adjustable balance system
  7.  Once this (stage 1) has been completed, move the OTA to a fully
    "nose-up" attitude - the Dec axis doesn't change position - and make sure
    that, once again, the Dec clutch is fully loose
  8.  Move the nose of the OTA over, very slightly - no more than a couple
    of inches or so - to one side (N or S, again test both)
  9.  Does it stay there, return, or fall further away in the direction it
    was moved?
  10. Now this is where the use of a second dovetail bar becomes obvious -
    slide the previously attached weight left or right to counteract the
    off-balance movement. If you are lucky, this second adjustment will not have
    affected the first adjustment and you will now have fully balanced the Dec
    axis in two dimensions
  11.  Lock the Dec axis with the OTA in a 'nose-up' attitude again - Dec
    balancing is complete
  12.  Whilst holding the CW bar with one hand to stop any possible
    'run-away', loosen the RA clutch. Move the counterweight bar to the N-S
    position (counterweight bar 'horizontal')
  13.  Roughly balance the RA axis by moving the weights on the bar (inwards
    is always better, but needs 'more weight') until you have the RA axis more
    or less back under control
  14.  Start the 'gentle nudge' tests again, fine-tuning the weight positions
    until the RA axis can be left without fear of unbalanced run-away. This is
    the end of stage 3 and you now have the RA axis fully balanced as well
  15.  Move the RA axis so that the counterweight bar is N-S (OTA at the
    'top', and OTA horizontal - not that this really makes any difference). Lock
    the RA clutch
  16.  Fully loosen the Dec clutch - an un-modified EQ8 mount will tend to
    'fall' to the side of the motors when 'nudged'
  17.  (again, DIY ingenuity may be required here) add weight to the
    'opposite side' of the casing from the motors
  18.  Adjust the amount, and position, of the added weight to balance the
    motors - so that there is no run-away
  19.  Unfortunately, at this point you have effectively added weight to the
    same side of the RA axis as the counterweights. You will almost certainly
    now have to repeat the counterweight balancing steps to adjust out this new
    imbalance but that is relatively easy and you will nor need to ever make
    this compensation again because balancing the motors only ever needs to be
    done once, and is totally unaffected by the OTA
  20.  As a 'sanity check' go back and repeat all three balance tests - just
    to make sure that one adjustment has not affected another

IMPORTANT: When you are making 'nudge' offsets to check balance - they need to be 'small offsets' (just a few inches to one side or the other).

Remember you are making fixed-location, static balance tests. Move the OTA,and only release it when it is 'stationary'.

Go too far from the start point in each stage, and you are trying to balance the OTA under the influence of more than one set of conditions.

Once you have got all three axes balanced, you should be able to 'throw'
your OTA around - with confidence - knowing that friction will safely bring
it to rest.

Finally, refit all the cables. This action - alone - is all the
'pre-loading' that you need for either/both of your axes. (Well, it has
certainly always been the case for me).

Just the 'drag' of the cables should be enough to keep you gear train fully
meshed at all times (although 3D balancing does nothing to remove the actual 'backlash' of the gear train, i.e. how much an axis motor needs to turn in a changed direction before all of the 'lash' in the gear train is taken up, with all gear faces and surfaces now fully engaged in the opposite



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