Ultrawide lenses for astrophotogaphy have a difficult job. Light from widely diverging angles must be catured and bent to come to focus in a flat image plane. My first ultrawide lens was the bargain Rokinon 12mm f/2.0 NCS CS Lens for my crop sensor E-mont Sony a6300. I've taken some of my favorite images with it. Even with a 99 degree corner to corner FOV star images were acceptable to me in the corners.

For my full frame a7 iii camera I started the Samyang/Rokinon 14mm f/2.8 ED AS IF UMC Lens. This gave me an amazing 114 deg. FOV over a full frame sensor. However I found that I was happy with the star image quality in the corners and routinely cropped these images.

Recently Sigma announced a 20mm f/1.4mm DG DN Art lens optimized for the Sony full frame E mount with some well thought out features for astrophotography. I prefer a manual focus lenses for astrophotography, but the Sigma has even better feel and control in their focus by wire. It also has a manual focus lock swith to prevent focus drift while pointing skyward. The profile of the front of the lens barrel is designed to prevent a dew heater strap from slipping into the field of view. After arriving (thanks for the generous family support), we had a week of nightime clouds here in Austin. Finally, my first light image of Juipter in the eastern sky is below, taken just after midnight 2022-09-05, 94 degrees corner to corner; it fully meets my expectations.

This was shot with the Rolei AstroKlar CLS filter, 5 sec. at f/1.4. I also shot with the bare lens and the amazing Sigma ceramic glass protection filter. All looked great. The Sigma protection filter is invisible with no reflections until you are nearly parallel to the glass. It is no doubt effective against grackle poop. The four times increase in light gathering and the much reduced aberrations in the corners, make shooting wide open without cropping a no brainer with this lens. It is a big step up from the Samyang 14mm f/2.8.

Also thanks to B&H for getting this to me in a flash after Austin's local camera shop bungled my pre order.