Photo pills may be a paid application, but many astrophotographers will agree with me when I say that it is well worth it. This mobile astrophotography application is a treasure trove of different tools to help you plan your exposure time, camera settings, field of view and composition. It has an augmented reality feature that is particularly handy for finding out exactly where the moon or the Milky Way will be positioned.
Many astrophotographers have used this application to create time lapses, photograph star trails, and more recently, to create some of the best images of comet NEOWISE.