Re: GPS on a German Equatorial Mount

Joe Zeglinski

Hi Brian,

Thanks for the opportunity to explain this further - and no, I am not
favouring users in either hemisphere.

I don't know of any GOTO mounts that still have a N-S switch to change the
polarity of the drive motors, according to hemisphere where the scope mount is
used. I think the N-S switch is necessary for the cheaper "Digital drive
controllers", which track using crystal oscillator drive circuits, but NOT
computerized GOTO drive systems. In the latter, the computer firmware has the
capability to reverse the servo motor polarity, or even stepper motor phase
sequence, based on the user's supplied site coordinates for Latitude. If you
enter North xx degrees into your Gemini, for example, then the motors drive in
one way, and entering S xx degrees for your site, the Gemini tracks and slews
in the other way.

I believe the AP's GTOCPx controller N-S switch is a carry over from the
original AP900 SMD or QMD which were "pre-GOTO", and simply based on crystal
oscillator tracking and slew circuits. Since they had no computer, there was
no way of reversing the polarity of the motors without a N-S switch. That was
true for all similar mounts, decades before that, an is still true if you buy
a Losmandy "non-GOTO" basic G11 mount - it comes with a "Digital Drive
controller" - which does have a N-S switch. The Gemini obviously does NOT need
or have one. The Gemini adjusts it's servo motor drive polarity on the user
telling the unit where he is standing - north or south of the equator.

Now, with the AP GTOCPx, I would wonder why you need to tell the
controller where you are TWICE - once in the site entry (N or S latitude), and
still have to be mindful to set the mechanical switch to the corresponding N-S
setting. This is error prone duplication, and I wonder what would happen if
you only made ONLY one of these two settings, and I can't think of a single
instance where you would want to do that.

To me, it is obvious that this N-S switch is an oversight, a remnant from
the very first controller design, that somehow has been ignored in GTOCPx
design. At the very least, AP should cover the N-S switch hole on the panel in
their current stock, and fix the firmware to do the N-S motor polarity
switching, based on Latitude.

Even better, cut the traces to the switch and patch them into the power
socket - that way it can find a much better use as an ON/OFF switch, which
many of us would like to have - of course, this old switch would have to be
replaced with a high mechanical quality, LED lighted, switch, to avoid other

If I missed something in this reasoning, I would appreciate hearing a why
the mechanical N-S switch can't be replaced by a firmware change, to use the
site's Latitude setting instead.


----- Original Message -----
From: "Brian Coote" <>
To: <>
Sent: Sunday, June 17, 2007 12:23 AM
Subject: Re: [ap-gto] Re: GPS on a German Equatorial Mount

Hi Joe,
I'm curious why you feel the N/S switch is redundant.
Is this because no one uses a GEM in the Southern Hemisphere?
If you point a GEM set to north at the South celestial pole it rotates from
West to East which isn't as useful as you might suppose <g>
Removing the redundant N / S switch isn't a good marketing idea.
----- Original Message -----
From: Joseph Zeglinski
Sent: Saturday, June 16, 2007 11:54 PM
Subject: [ap-gto] Re: GPS on a German Equatorial Mount

There may be more desirable improvements, like USB, and "nice-to-haves"
like a power on/off switch, perhaps a nice compact "AP embossed" roto
travel case, mount levelling bubbles, (or getting rid of that redundant
North-South switch on the panel). An on-going "improvements suggestions"
on this group, might be useful.


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