Worth it, though, even though I've never tried astrophotography with a phone.

What you have to remember is, when you start getting beyond fifteen or twenty seconds on the exposure, you're going to start getting trails. I tried doing thirty-second exposures (the maximum with unmodified firmware) on my Canon 60D, and the star trails just weren't worth the extra number of stars that I was picking up on the sensor. And, since my choices were to ratchet down the exposure time, buy a tripod that could match the rotation of the Earth, or plead with the almighty to stop the rotation of the Earth for the duration (the book What If? tells us this would be bad), I went with shorter exposure times.

If I recall, the best solution is to open your aperture to its widest value, set the focus to just shy of infinity, set it on a tripod, and either trigger the shutter remotely or set it on a timer to start the imaging at some point after your hand has cleared and the tripod has stabilized. Pretty simple.

There's one more thing, and I don't know how it affects phones, but long exposures can create a fair bit of heat on the sensor, and that can muck up your shot. Given that the same sensor can shoot 4K video at however many frames per second, it's probably not the problem that it was, but you never know. I'm sure the hardware would probably kill power to the sensor if it sensed a thermal overload, but I've never looked into that.