Still waiting to see if Sigma corrected for both Coma and Astigmatism in the 14mm 1.8. Their past fast Art glass all have had considerable coma which is very noticeable wide open to around F3.2. (20mm, 24mm 1.4s) Their 35mm 1.4 is still the gold standard for night work IMO when you can use it as it is basically coma free.
The Samyang 14mm (when you get a centered lens) has a very low coma aberration also. Sadly the Irix 15mm F 2.4 is has Strong coma until around F 4.
Coma will effect exposures for nightscapes where you are not working with star trail work.
Still waiting for www.lenstip.com
to get a Sigma 14mm 1.8 for their standard testing. They do a great coma and astigmatism test.