I've never tried it on astro images, but maybe look into
hyperfocal distance focusing. Normally the range of acceptable focus is limited by focal length, aperture size, and circle of confusion (sensor).
Hyperfocal distance is an optical property where you can set the focus to a particular distance (H) and everything will be in focus from H/2 to infinity.
More here:
https://www.photopills.com/calculators/hyperfocal-table
And here is a calculator for you to figure it out for your particular gear:
https://www.photopills.com/calculators/dof
Looks like for your camera and ED80 the distance to focus would be around 3-3.5km. Might take some trial exposures to nail it.