Alexander,
I had (still have) two same lenses from different years (the first one with only 6 digits serial number
). I used these lenses for nearly 13 years. While I really loved them for landscape photography (very sharp, no distortion, awesome beautiful sun rays around spot lights at f11) they are not good for astrophoto. This lens has really bad coma towards the edges at f1.8 - f2.8. Plus general focusing issues at open aperture because of discrete focusing system (cannot be stopped at any chosen point but only at some points) inside (not the case for landscape photo at closed aperture, but real issue with astrophoto) and backlash. This lens has a "screwdriver"-type focusing driver which is not very accurate anyway. Coupling with discrete focusing this becomes a real issue. At some point both my lenses broke because of plastic gear inside.
I spent fair bit of time looking for non-expensive alternative and ended up with Tamron SP 45/1.8. This is absolutely fantastic lens for astrophoto - no coma at wide open, very sharp, and not so expensive.
So if you want to use it for landscape photography - keep it. At f8 and smaller it's a fantastic lens that costs only $150, produces images with better quality than from lenses for $1000.
If you want to do astrophoto with it - better to change it to something else.
Regards,
Ilya