Warning: I stopped shooting my astro photos at f/2.8 and my keeper rate tripled
I used to chase the lowest f-number I could get, figuring fast glass meant more light and that had to be better for dim targets. Last month I shot the Orion Nebula over two nights in my backyard near Denver, once at f/2.8 and once at f/5.6, and I got 4 sharp subs out of 40 wide open versus 31 out of 40 stopped down. The stars at f/2.8 were soft little blobs with purple halos, and I kept blaming my focus and my mount when it was really the lens all along. I still use it wide open for the Milky Way because the trailing hides the mess, but for anything I plan to stack? No way. Does anyone here actually get clean round stars at f/2, or are we all just pretending it works?