[OM] reflex lens advice needed

[William Clark]

Hey all.  I just joined this list, seems to be good.  I am trying to decide
if a reflec lens for my om-1(n) is a good idea.  Anyone with experience with
one?  I plan to use it for astrophotography (I do have a telescope) and for
land-based "fun" use.  I am trying to decide between:

1.  Zuiko 500mm reflex f8
2.  Sigma 600mm f8
3.  Sigma 500mm f7.2 APO

Any insight would be helpful.

Thanks,
-Bill