Re: GPS on a German Equatorial Mount

Dean S

I have the Gemini on my MI-250 that I bought while on the long wait for a AP900 a couple years ago and have decided to keep at this point. I use an external gps that plugs into the Gemini at start up, and automatically sets the time and location. Handy and easy to get the right time, but I still have to know my UT offset.

For a mount (permanent)that does not have an internal clock, like my LX200gps scopes, having the gps get the time is great for the accuracy and will get my first star on the chip most times from a parked position. My Gemini has a built in clock and I only use the GPS if I have moved to a new location and again mainly for the time as location really doesn't matter that much if you are polar aligned correctly.

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.

And cost is an issue like Joe said, when I upgrade to a 1200 I hope not to have to mortgage my house :)

And I really like the idea of an OnStar like button directly to Roland's cell phone :))

(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

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 setup.

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 cases.


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