Re: GPS on a German Equatorial Mount

Joe Zeglinski

Hi Marj,

Please ignore my previous, obviously confused reply. I was too tired to
think straight. Of course, I was confusing "calculating" local sidereal time -
not setting up the UT entry. Without a computer, LST does require knowing
one's precise longitude within a time zone, and GPS would be an occasionally
used asset. However, GPS coordinates can just as easily be obtained clicking
on Google Maps, for the intended destination observing site.

I have considered GPS, but the portable one's are getting so cheap in the
local electronics store, that I wouldn't want to waste money and firmware
space for such a trivial item on-board. Even my digital Kodak DCS PRO SLR/n
camera has an option for a GPS module inside. If you fish, you might already
own a GPS based fish finder, which can be used for mount data setup as well. I
don't know if it is possible yet, since I don't own one, but cell phones
should be able to display GPS coordinates. New units are required, by the
authorities, to have GPS capability. If not displayable, then placing a call
to a GPS capable site, should be able to text you back (or by automated
voice), the coordinates where you are calling from - perhaps that could be
made a "geocities or Yahoo" capability. It would be nice to use the cell phone
for GPS info at setup - which in most cases, for a new observing site, would
only be a one time requirement anyway. Then again, a car's GPS navigator
would suffice as well, with dual use - if you don't trek too far through the
woods, from where you parked. Or, press your ON_STAR button, if you are
already paying for that service, and ask the operator: "Where the heck am I ?"

However, in the end, I have found that before a trip, I can Google Map
zoom down - even to the nearest tree - and use those GPS coordinates when I
get there.

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 moulded
travel case, mount levelling bubbles, (or getting rid of that redundant
North-South switch on the panel). An on-going "improvements suggestions" file
on this group, might be useful.

Otherwise, there isn't much else to improve on such a fine instrument like
my AP900. You also have to be cost conscious - too many add-ons, and the mount
will be even less attainable in most amateur's lifetimes.


----- Original Message -----
From: "Joseph Zeglinski" <>
To: <>
Sent: Friday, June 15, 2007 8:28 PM
Subject: [ap-gto] Re: GPS on a German Equatorial Mount

Hi Marj,

Just one question about "time precision". I think the GPS unit might give
you the EXACT time at your precise location - give or take 10 metres I
believe, for the non-military GPS. If so, the GPS time won't be in agreement
with your usual "Time Zone" which is one hour wide. If your observing site
is right in the middle of a time zone, then they agree, but if you set up
+/- 30, minutes away from the centre of the zone, doesn't the GPS give you
that exact time, rather than the time zone standard time? So, while the UT
is always correct, the rest of the world is on a one hour wide approximation
of actual time. There are only 2 correct time references - GPS time, and the
Right Ascension of the star overhead - both are accurate for the site
geographic Longitude.

In the olden days, B.C. (before high speed electronic confusers), we
would calculate time for the geographic spot, with a lot more care, perhaps.
It would be interesting to compare the time displayed by a GPS unit and the
observer's local time. Then again, the GPS works only in UT, so site
coordinates are irrelevant, for this.

Consider this. An AP mount owner goes out and has to enter the GMT into
the controller. Not having a GPS unit, he looks at his wrist watch, adds or
subtracts his Time Zone correction, and declares this to be the exact UT.
However, he may be +/- 30 minutes away from the centre of the zone, so the
setup is way out.

The question here is whether we are supposed to add the time zone
Latitude delta ourselves, or does the AP's GTOCP3 controller look at the
observer's site coordinates and compensates for the "site's longitude
difference" for us? I don't think this is stated anywhere in the manual. I
suspect that the GTOCP3 does not compensate, so the GPS based UT would be
exact, even 30 minutes different from the standard non-GPS based mount

Marj, at the very least, the AP firmware should make the corrections for
the user entered time zone based, UT. It's such a hassle to pencil in the
local time zone delta, to the wrist watch based time. That's where the GPS
would be valuable.

But ... I don't know if all that exactitude really matters, in most


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