Mount a phone rigidly to a telescope or camera lens and AstroHelm photographs the sky, matches the stars against a catalog and shows which way to move the scope. Because it reads the stars rather than the compass and gyroscope, it works without a GoTo mount and offline, and it was built for city skies, though it needs at least eight visible stars. The app is on the App Store and Android; plate solving, the Moon and planets are free, with one-time iOS upgrades at $9.99 and $29.99 for bigger catalogs.