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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
itself)
- 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
matter)
- Tighten the RA axis clutch
- 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)
- Move the nose of the OTA over, very slightly - no more
than a couple
of inches or so - up or down (test both)
- Does it stay there, return, or fall further away in the
direction it
was moved?
- (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
- 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
- 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)
- Does it stay there, return, or fall further away in the
direction it
was moved?
- 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
- Lock the Dec axis with the OTA in a 'nose-up' attitude
again - Dec
balancing is complete
- 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')
- 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
- 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
- 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
- Fully loosen the Dec clutch - an un-modified EQ8 mount
will tend to
'fall' to the side of the motors when 'nudged'
- (again, DIY ingenuity may be required here) add weight to
the
'opposite side' of the casing from the motors
- Adjust the amount, and position, of the added weight to
balance the
motors - so that there is no run-away
- 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
- 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
direction)
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