Sigma 35mm F1.4 DG HSM Art
Below I have compiled the centre frame, and extreme edge performance of the Sigma 35mm f1.4. Shot at f1.4 through to f2.8.
These are all 100% jpegs, if you right click "open in new tab" you can inspect/download them at full size. They cannot be viewed full size within the PentaxForums interface.
First, for reference, is a single frame at f1.4, then a stacked shot at f1.8, with some tone mapping in PS (which would in theory emphasise lens aberrations).
The long and short is this lens is excellent in the centre even wide open. I've only shot its once for astro, but in the future, I will use f1.8 if I'm stitching a panorama, or f2.2 if I want to use a whole frame in isolation. There is noticeable astigmatism in the corners (tangential + longitudinal), but it improves well from f2.2 onwards. The lens has no issues with spherical aberration or a curved focal plane, which is the most important performance feature for me in an astro lens.
The next time I have clear skies, I will repeat this test with the Sigma 18-35mm f1.8. I see there are some samples of this lens earlier in the thread, but my anecdotal experience is this lens is much better than is being demonstrated there. Similarly, this lens is exceptional in the frame centre even wide open, and probably has equivalent or better edge performance at f1.8 (albeit it is using a smaller image circle, so its edges aren't that far from the centre).