When a user views a deep sky object of known magnitude and has a location entered to see the best observing time, it would be useful to compare it to the minimum visible magnitude from that location. I don’t know of a specific API offers this information, but if one exists, it would be a useful addition to Telescopius.
I couldn’t agree more! I’ve been looking for this kind of information too, so if anybody knows a good source of light pollution information we can use, I can incorporate it into Telescopius.