----- Original Message -----
Sent: Friday, June 01, 2001 1:07 AM
Subject: [M]: Goto accuracy and slew speed


>........realized tonight that if
> I decrease the maximum slew speed from "8" down to "5" then my pointing
errors
> decrease to around 5 arc-min for medium slews (< 90 degrees) and around
5-15
> arc-min for large slews. Has anyone else experienced this increase in
accuracy
> at slower slew speeds? It makes sense that this would increase accuracy
but
> I've never seen this mentioned anywhere. The pointing error that I get at
high
> slew speeds is almost entirely in RA and almost none exists in DEC. Also
the
> error is always because the RA drive overshoots the target coming from
either
> direction.........

The fact that the problem is only in the RA and always
involves overshoot is consistent with a situation where the microcontroller
is
receiving too few pulses per revolution of the RA encoder wheel at the
higher
slew speeds. As a result, the controller tells the telescope to slew an
additional
amount in order to count off the number of pulses it "knows" it should get
when
the motor moves the scope by the amount that you commanded.

I've not seen previous discussion of that kind of failure on mapug, so it is
just
possible that you're breaking new ground. I certainly hope so, because
fresh
problems are the life-blood of mapug, not to mention salvation from time
zone
confusion. Whenever the time zone questions and answers come up, I start
worrying about how to remember whether it's spring back, fall forward, or
fall
back, spring ahead. No amount of logical analysis (by me, anyway) can solve
that
problem, but I can't help trying, and I know perfectly well that I'll just
waste a
lot of time and then finally give up and humiliate myself by asking someone
which
way it's supposed to go (usually someone far less clever than myself). When
they
flippantly toss off the answer without even thinking, it's the other shoe
dropping,
another little stab in the gut.

Anyway, at high motor RPM, the frequency generated by the encoder
slots is probably around 10 kHz (just a rough guess). You try counting that
fast and
see how well you can do. But seriously, if the encoder circuit board is out
of tune,
meaning the LED's are dirty or whatnot, or the bias pots are wrong, or
there's a
bad solder joint, then some of the pulses won't make it through the input
chip as
bona-fide pulses. High frequency pulses are more likely to get lost because
capacitative leakage differentially attenuates higher frequencies. It's
sort of like
fast pulses are shorter pulses, and short things are easier to lose than
long things.

Sounds like a warranty type motor/main board replacement is the answer,
unless
you want to dig in the archives for DIY info about the encoder circuits and
how to
tune them. On the other hand I could be completely wrong in my analysis.
If it were my scope, I'd stick an oscilloscope on the encoder to check it
out.
Unfortunately, while the encoder mini-pc board is likely to be the problem,
it's
also possible that the main board is undercounting pulses coming from the
encoder
for some reason, and who knows where that might happen or how to prove it
other than by swapping out the main board. Have you talked to Meade about
it?

By the way, it makes no sense at all that operating at high slew speeds
would impair
the accuracy of a GOTO with this scope (when it is working properly, that
is!!).
The control mechanism is as digital as a DVD, and there's no significant
slippage in the
gearing. There will be total conservation of pulses in such a system, so
the same
accuracy of GOTO would be obtained throughout hours (or days) of slewing
around
(at any speed) after a single alignment operation. If an lx200's GOTO
accuracy falls
off as the night wears on (without changing the alignment at all), then
something is
wrong with the scope's control system, IMHO. Or else the Earth is starting
to spin
differently than it has for a while.

Rob P
Pittsburgh PA

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