[APML] APML Gemini GoTo and Autoguiders??? - Update

["William R. Mattil" <wrmattil@ix.netcom.com>]

Hi,

I thought that I would share the results of some additional testing and
some dialogue that took place between Rene and myself regarding this
problem.


Per Scott Losmandy's request I removed the Dec motor, set tvc = 255 and
experimented with hitting the arrow keys on the ST-4 (simulates a
request to move) and watched the Dec Motor gear. Switching back and
forth between directions I could see the tvc parameter work its magic
... *most* of the time. Occasionally it would not activate the tvc upon
a reversal in Dec. This was very reproducable and after giving Rene this
information he responded with:

-------------------------- included message-----------------------------
Rene writes:

I consider this the clue... what's happening is that a move into one 
direction is stopped (since the contact was no longer good enough to 
allow the necessary amount of current flowing). Once the contact was 
reestablished it was considered a move into the _same_ direction as 
before and TVC was not applied again. So there are some contact
problems.

Are you electronically inclined? The Gemini board has a protection 
resistor R42 (100 Ohm) at the AG common pin, in case users use a wrong 
cable or similiar. The R42 resistance does not matter in normal cases, 
but adds up with the relay contact resistance. Since you have a ST-4 
and not a TTL output autoguider, you can bridge R42 with a short piece 
of wire without introducing any riscs. The resistor R42 is located 
directly above the AG socket, the leftmost one. Hopefully this will 
bring us back into the "safe zone" of the contacts again. FYI, the 
pullup resistors R5..R8 (carrying the +5V voltage while the contacts 
are open) have values of 3.3k Ohm, so even a 1k closed contact should 
work.

--------------------------- End of Message
------------------------------------

So at this point I felt vindicated as I knew there was some problem that
was *not* excessive backlash in Dec. So I jumpered around this resistor
and retested tonight and inital results are quite favorable. At least at
this point I can get the ST-4 to calibrate. I won't say that the
situation is resolved as I still need to experiment with the tvc and
C2,H2 settings but I can at least calibrate. Tracking in Dec is still
below what the original controller could do and I suspect the main
culprit with this is the low guide rates offered.  But time will tell
after I experiment with different settings.

Any new purchasers of a Gemini that plan on using an ST-4 should file
this away if needed later. The Gemini circuitry was changed with the
addition of accepting TTL autoguider input and these changes apparently
affect the relay input from ST-4's and STV. Extremely short relay
closures will get ignored by tvc. Jumpering this 100 ohm resistor out
improves this a great deal.

Prsonally I find it curious that I was the first to run across this
problem. Thats a position I usually try my best to avoid.

I would like to thank Paul (Losmandy List)for providing much useful
input, and Kent
Kirkley (from APML) for assistance with his STV, proving that it really
wasn't the ST-4(s) after all. And of course Rene and Scott for accepting
the possibility that something unforseen had arisen and helping to
resolve it.

Regards

Bill


PS: Initial tests done with tvc = 005<--->025 and C1 = 10, C2 = 20, H
param's = 0 Guiding speed of .8x

-- 
William R. Mattil	|                              
wrmattil@ix.netcom.com	| http://celestial-images.com      
(972) 399-4106		|
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