[Kromm]

Speaking here as a physicist who had zero courses in astrophysics but knew quite a few people who worked with big dishes and mirrors:

Theoretical astronomy would be best handled as Physics (Astrophysics). Cosmology, too, would most accurately be a Physics specialty. These overlap some, but not enough to need special rules – cosmologists are much more like theoretical particle physicists or string theorists than anything else. That said, the Astronomy skill can be given a purpose at high TLs . . . treat it as an equipment-operation skill that replaces Computer Operation, Electronics Operation, Navigation, Observation, etc. for any and all gear that's pointed at the heavens. Different kinds of gear would be optional specialties of that skill, not of Physics.