I have a Nexstar SLT 102mm Celestron Refractor. It calls for a 12 volt Celestron power pact. I have a 12 volt car battery charger. It has a plug-in I can use. But it puts out 13.4 volts instead of the 12. Is it safe to run the motors on this?
Yes, it's safe. All specified voltages, both AC and DC, are actually voltage ranges. 13.4 is very close to the normal resting voltage for a fully charged 12v battery.
Car jumper battery pack? Those things are great, pretty cheap and last a while. I got one for my now departed LX200GPS that I just sold for parts. I plan on keeping the battery in the car for safety. Scopes pull next to nothing in amps.