I am adding a few instructions for optimization.

- turn off HDMI, to save a little power.

- turn off LED after we send the time to the hand controller (and location)

- change to a RO filesystem, so a hard power cut should not cause any havoc on the SD card and increase the lifetime.

I have decided to setup two boxes. One PI zero W for observing from home:

• Wi-Fi via the home network. (and of course you have to make sure you have coverage where you are using the telescope.
• No GPS. I have not yet found a case for the PI where everything fit's in. So the location I am sending to the hand controller is fixed to my home location.
• Time is synced to NTP public NTP servers. 
• I will use power from the telescope mounts AUX port with a RJ12 connector. (not yet finished)

Currently I have the time set on the HC after less than 60s.

Another unit, most likely a PI 3 B+ or PI 3 A+, as the cases have a little bit more space to add the additional components. Or maybe a PI Zero in a case of a PI 3. 

• Wi-Fi via home or if out of reach enable Hotspot.
• With GPS to fetch time and location
• Power from the telescope mounts AUX port.

I dropped the idea of adding a RTC. NTP is pretty fast, and with the GPS I will have a super accurate time.