Re: GPS on a German Equatorial Mount

Joe Zeglinski

Hi Dean,

Don't quite understand your comment about needing GPS mainly during the
"first" set up. The question is how long is it between your observing
sessions, that you can rely on not using the GPS update.

I have a Gemini as well as the AP900 GTOCP3 controller, and yes both have
internal clocks which continue to run even if the power is disconnected.
However, I question the long term accuracy maintained by either the Gemini or
GTOCP3 - their clock modules use a limited number of clock count bits, or
precision, so if you don't use the scope for a month, the internal clocks
might be a bit off. I haven't actually measured the drift, yet, but that is
about the same as your digital wrist watch. A GPS provides perfect time, every
time, since satellites rarely slow down, and they back each other up. A
caesium clock might be better. Since a laptop is quite commonly used with such
mounts anyway, for some it might be simpler and cheaper, as has already been
suggested, to sync up the Gemini or AP with a good internet time source, like
the National Bureau of Standards, so a GPS isn't essential, though convenient,
if you accept the extra cables, in return for that. Even if going on the road,
without a laptop, this sync could be done at home, prior to a trip into the
hinterland, and these internal controller clocks will then be sufficiently
reliable for another while.

I would think that once one invests in a GPS, it should always be used at
start up of every session. Even if the mounts are powered up every day, the
internal clocks steadily still slow down by some very small amount.


----- Original Message -----
From: "Dean S" <>
To: <>
Sent: Saturday, June 16, 2007 10:32 AM
Subject: Re: [ap-gto] Re: GPS on a German Equatorial Mount

So for me the GPS is mainly for time during the first set up, then the
internal clock keeps time well enough that I my gotos are accurate the next
time I start up from park.

Do the AP mounts have internal clocks? That would be important to me.
(future owner)
----- Original Message -----
From: "Joseph Zeglinski" <>
To: <>
Sent: Saturday, June 16, 2007 9:54 AM
Subject: [ap-gto] Re: GPS on a German Equatorial Mount

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