I love the F/1.8. I'm currently using it to do a whole sky (well, at least what is visible at various times of the year for my location) panorama project. You do need to stop it down quite a lot to get sharp stars right to the corners. For example: F/2.5
http://greg-gibbs.deviantart.com/art...epro-317071198 - stars still showing elongation at the corners.
F/4.5
http://greg-gibbs.deviantart.com/art...ross-296353826
Hope this helps.
Cheers
Greg