DPReview has posted an even more in-depth look at how the Sigma 14mm f/1.8 DG HSM Art series lens works for astrophotography.
From DPReview:
As I mentioned, my go-to lens for wide-field astrophotography is the Nikkor 14-24mm F2.8G ED. If I needed to pick just one lens to photograph the night sky, would I trade it for the Sigma 14mm F1.8 DG HSM Art? The answer is yes. Although I often use the Nikkor lens at different focal lengths to photograph buildings and monuments (it's the perfect lens for cathedral interiors), I rarely use it to photograph the sky at focal lengths others than 14mm. When taking single shots of the night sky (something I often do while other cameras are shooting time-lapse sequences) the extra 1.3 f-stop would allow me to take shorter exposures and use that gained time to creatively experiment with different compositions and angles. Read the full story
It looks like if you're into shooting the night skies, this new Sigma is going to leave you very impressed.
Sigma 14mm f/1.8 DG HSM Art: B&H Photo | MPEX | Park Cameras (UK)