Hi,
I made two new pages where I compare the stars from the Pentax lens and the Sigma lens. It's from center and the extreme corner. It's interesting to study these photos, in some cases the old Pentax non APO looks sharper then the more modern Sigma APO lens. And the Pentax is at full aperture (f/2.8) and the Sigma is stopped down to f/4.
Have a look at page 7 and 8:
http://www.astrofriend.eu/astronomy/...vs-pentax.html
What I know the Sigma APO 150 mm f/2.8 is very good, even for astrophotography when stopped down one step.
/Lars