Hi Greg,
I can't use the Sigma at f/2.8 with a full frame camera and get acceptable quality in the corners, most because of very strong vignetting.
Then it's more interessting to compare the two lenses where they perform at it's best.
The Sigma deliver ovesaturated stars at the same exposure because it's sharper and then concentrate the photons of a star to fewer pixels. I can take shorter exposures to bring out the color, but then I get problem with the weak details, more have to do HDR imaging as I did with the Pentax lens photo, one of them.
But this time I'm only looking at the vignetting and the shape of the stars at the edges. I will take more photos with the Sigma lens to find where it perform at its best. I had a small drift because of bad polar alignment in this case. I have overcome that problem now with a better view through the polar scope.
http://www.astrofriend.eu/astronomy/...iewfinder.html
My biggest problem as usual, there are clouds at the sky!
ps.
I had a look at your homepage, wonderful photos!
/Lars