> I take part of that back. I think I'm using USNO A2.0 for PinPoint, not UCAC3.

Most likely, yes. I just checked; it would be theoretically possible, I
think, to read a .bz2 file in a suitably "random" way that would let PinPoint
(or Guide) access some data from the middle of the compressed file. That would,
in fact, be very handy in some situations. But I doubt it's what PinPoint
is doing.

> So decompressing all the .bz2 files should be sufficient, right? All 360 of them...

Yes. If you don't really need the far southern files that cover areas
you can't see from your latitude, you can neglect (say) z001 to z080, thereby
leaving out declinations -50 to the south celestial pole. It probably won't
save you a lot, though.

-- Bill