Re: AP 1600, Universal Time and Connecting to Auxiliary Astronomy Programs for Control

Dale Ghent

So here's the deal: the mount CP deals only in UTC. All calculations involving positions of objects in the sky are done UTC. For the purposes of a mount's internal operations, UTC is the only thing that matters.

When you set the timezone on the keypad, you are essentially just configuring the keypad's display and input of UTC time as local time. The mount itself doesn't care that your offset from GMT is now -5 instead of -6. That factors into none of its operations and calculations. What it does factor into is how your keypad displays time, and how it converts time that you input to and from UTC. The former - displaying the same time as your local time - is just for convenience. The latter - inputting your local time - is critical for the mount to derive the proper UTC time if you were to program the mount's clock using the keypad. So if your local time is 18:45:00 and you were to program your mount's clock using the keypad today, it would definitely need to know your current offset from GMT so that the proper conversion to UTC can take place (that it would be 23:45:00 UTC).

The reason why you have to manually set the GMT offset in your keypad every time DST begins or ends (or if you move to a different timezone altogether) is because the keypad lacks its own timezone database that it can use to do this automatically. You might ask "well how hard would it be for the keypad to have one so that I don't need to do this". It's a valid question, but the reasons are fairly basic. The timezone database is large and would occupy a non-trivial amount of the keypad's very finite amount of static memory. It also changes quite often as various countries and locales change their DST observances or, in rare cases, which timezone(s) they are in. This would demand that A-P issue several firmware updates a year in order to keep up with these changes.

As for the computer and accessing the mount through the ASCOM driver, this changes the situation in terms of mount time management. In this case, it is presumed that your computer's clock is reasonably accurate as any modern OS will automatically sync the computer's clock to an internet time source at least once a week or more often, assuming it has internet access. There is a setting in the A-P ASCOM driver to sync the computer's clock to the mount. This should always be on if your computer's clock is reasonably accurate. The ASCOM driver itself will then program the mount in UTC time directly in this case. Despite your computer's timezone being set to US/Central, the OS itself still operates in UTC internally. Because your OS does have a maintained timezone database, it can do the display and input conversions between your timezone and its internal clock, which runs in UTC.

So, the only reason why you would want to run your computer in UTC is if you prefer viewing time that way. Even if you set the display of time on your PC to be in your local timezone, the mount is still going to get programmed with the proper time in UTC. It makes no difference to the mount.

As for the keypad, you can also just leave it with a timezone offset of 0 so that it displays and takes input in UTC. However if you want it do display and take time input in terms of your local time, you will need to manually set the proper GMT offset whenever DST starts and ends.

On Mar 16, 2021, at 09:13, Jack Huerkamp <> wrote:

I use SkyTools 3 and SkyTools 4 to control my AP1600 mount using the ASCOM driver. But every 6 months, I was having to switch the AP keypad from standard time to daylight savings time and then back again 6 months later. It was suggested to me to set the AP keypad to UT and also setting the computer to UT to avoid have to mess with keypad settings every 6 months. I did so and can control the mount using the keypad.

I then tried configuring my observatory location, but when I connected the mount to SkyTools and did a slew to the Sun this afternoon, the scope tried to aim into the ground on the southwest side of the pier. I need to know how to set my location in SkyTools now that the mount and computer are set to UT. I am in the Central US time zone.

I would appreciate feedback on this as I would hate to have to go back to setting the computer and AP keypad to the current local time.

Yours truly,

Jack Huerkamp

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