Ad Astra Observatory

There are two adjustments you can make on each axis to minimize backlash,

the play in the worm mount and the meshing of the worm to the axis gear.

The worms on the EQ series mounts are nicely supported on bronze bearings
perpendicular to the worm, but the thrust bearings on the ends of the worm
are just rubber o-rings, leading to a LOT of backlash as the o-rings compress.

If you don’t want to take the mount apart to replace with Teflon washers,
you can still improve the situation.

First you want to loosen the mesh between the axis gear and the worm

itself. On the worm assembly there’s two large hex bolts with a smaller one
in the middle (upper picture, there’s another two close by without the
small hex bolt in the middle – they hold the worm cover on, leave them
alone.) They form a push-pull assembly meshing the worm against the axis
gear. Loosen (but don’t remove, the worm assembly will come off) all three.
If you grab the axis and gently push on it, you should find that it wobbles
now, meaning that the worm is disengaged somewhat from the gear.

Disconnect your motors, then loosen the lock nut on the side of the worm
housing. The inner nut presses down on the o-rings. If you back it way off
you’ll be able to grab the brass rod and push and pull on it and feel it
move back and forth. You want to tighten the inner nut down until it
compresses the o-rings and mimizes the amount the worm can move back and
forth. You can tighten it down so much that the worm won’t turn. You want
to tighten it enough that the worm is hard to turn by finger (which is why

we unmeshed from the gear, so that you aren’t trying to move the whole
axis, just the worm). Fiddle around until you can’t really push/pull on the
worm but you can still turn it. Lock down the lock nut.

Now you’ll re-mesh the worm and the axis gear. Again, you want it tight
enough that there’s no play between the worm and gear, and yet not so much
that the worm binds and you can’t turn it. Adjust the push/pull bolts on

the worm assembly, manually trying to rotate the axis. At first there’ll be
some play, but you’ll eventually get to the point where you can’t really
feel any.

If you get all this stuff adjusted right, you should find your backlash
goes WAY down and you don’t have so many oscillation problems.

By the way, my guess on the horrible black “goo” that’s used to lubricate

these mounts is to make up for the relatively poor mechanical tolerances
inside i.e. the mount is actually machined kind of nicely inside, but by
using the goo they don’t have to carefully manually adjust things at
assembly time, just whack it together and count on the goo to cover up
looseness.

I re-lubed and adjusted my EQ3 and I’m getting +- 4 arc-sec tracking when
lightly loaded and making adjustments every second, which is frankly a hoot

given how much I paid for the mount…