Vignetting and coma on Nikon 20mm f/1.8

protik

Hi guys

I have just picked up this lens yesterday and wanted inquire if you guys think this is acceptable level of vignetting for this particular lens (no filter/hood); it's barely visible on the first image however when viewing on LCD I could notice rainbow like circular bands near the edges (is this normal?):

Vignetting (ISO100, f/1.8, 1/100s):

Lens profile correction on LR6 (this is pretty much what I saw when I took the shot):

-1EV:

Also, the clouds are particularly yellowish (hopefully due to ambient lights?) and I can't tell if plane near the edge is shaped like that due to coma or slow shutter speed (ISO1250, f/1.8, 1/2s), the stars didn't appear like this with similar settings in few other shots I have taken:

I am not sure if I should exchange this copy (meant to be used for astro) - any feedback would be appreciated specially if you have this lens; it's pretty sharp otherwise!

Cheers

spraynpray

Mine lasted one nightscape trip before going back to the shop.