The documentation mentions "Horizon" (local alt/az) and "Equatorial" (inertial RA/DEC) as typical settings for option "CelestialSystem".
I'm assuming setting "Equatorial" specifies a mean equator and equinox at the J2000 epoch, since I see no place to give an epoch for a true-of-date computation.
Are there other legal values for this option, such as "ECEF" (earth-fixed: https://en.wikipedia.org/wiki/ECEF)? My guess is No, again, because most such other systems would require some form of time evolution, and there is no obvious place to parameterize the option with a target epoch.
This is all we can handle at the moment. In the future we will add more support for astronomy. We are currently planning many updates to these areas, but we don't yet have a time frame for when they would be available.