Hello there,
I've been playing around with FG since a few months with the SpaceShuttle, and discovering (how to crash) other planes and helis as well!
Maybe the subject has been put forward long ago, but, as an amateur astronomer, I can clearly tell that the number of visible stars by night is not enough, and by quite a lot. Our current sky is looking like a quite polluted sub-urban sky, and that is not what the sky is at 10000ft or elevation, for sure!
I've explored a bit the code, and found simgear/scene/sky/stars.cxx, which confirms that we cut the visual magnitude at 4, a comment mentions that this is to keep the number of stars below 500. I played with the code, and I have done a few things:
1) Calculate the limiting magnitude by night using this paper:
https://arxiv.org/abs/1405.4209
which gives pessimistic/realistic estimates, and I get something around 6.2. Other estimation could be made as with the Bortle scale
https://en.wikipedia.org/wiki/Bortle_scale
which looks very optimistic, limiting magnitude could go up to 7.5. In fact, the main difference is due to the atmosphere which induces star scintillation, and may transiently boost the maximal magnitude visible by naked eye. Since we don't have star scintillation, I guess that is more realistic to use the result of the first paper, that took away this effect.
2) I have changed also the limiting magnitudes during astronomical twilight, based some ESO estimates of the sky brightness, and also during nautical twilight, down to -9 degrees for the sun position. The magnitudes I am finding there are also quite different from the ones we are currently using in stars.cxx. For brighter skies, the paper mentioned above is not applicable, and I've let the current settings as they were. Precise estimation there are not very relevant, only very bright stars are visible and they are not so numerous.
3) Our current star catalog is only a subset of the BSC, the BSC counts all stars up to magnitude 7.5. So I have changed our catalog by the original ones, containing all potentially visible stars, there are more than 9000 stars in total.
The result is closer to what I am used to in terms of number of objects, and the sky indeed contains thousand of stars. If I am not the only one interested in this, I'll be happy to submit a patch to the devel branch, but I have a few questions before:
a) Are we really limited in 500 stars for some reasons? There just dots in the sky, so I would say no and I have not observed any slow-down, but I may be missing something?
b) Do we really need the latin name of the star in the first column of catalog Astro/stars.gz? Most of the real stars do not have name, but are labeled by a number. I've no idea right now how the recover latin star names from astronomical professional catalogs that use HR numbers for instance, and that sounds like a pain...
I might dream in the future of a stellarium-like dome for the stars, dunno if someone has ever started such a project before?
Cheers,
Chris.