Stuart, all,
reading this thread with interest too
Have you thought of posting it in the ATM section too.
It might not come under the gaze of dabblers possibly?
Anyway, my 2c:
You need to make the worm stay centred and with zero
end thrust before you can even hope to quantify a meaningful
PE in arc sec. (error attributed to the worm anyway)
Also, having let this run with the (random)? periodicity of 75 sec
for months/ years, this could have also ground a slight random
error into the tooth facets of your worm wheel?
I hope you get to the bottom of it Stuart. Interesting stuff.
Steve