Anyone have some tips for nailing focus with a wide, fast lens?
I shot this a couple of weekends ago with my modded Canon 600D, Skyguider Pro, and Sigma Art 50mm f/1.4 at f/1.8. I thought I did a decent job of focusing... but nope. I got 108 90-second light frames at ISO 800, calibrated with 30 each darks, flats, and dark flats. Stacked and processed in Astro Pixel Processor.
Unfortunately when I stretched the image, donut-shaped stars were evident in the middle of the frame, and were smeared out into little comets at the corners. It isn't too bad scaled down for the forum, but at full resolution... it's just bad.
I really like how this lens performs when I can hit focus, but my success rate hasn't been great. My Bahtinov mask does not create large enough spikes to be useful at 50mm, so I've been focusing on dim stars with trying to maximize their brightness. I just don't think the way I'm doing it is precise enough below f/2. Everything looks fine blown up on the back-of camera LCD, but not once displayed on a full-size monitor. I also think the focus slipped through the night on this run, so I'll have to try to tape it down.
I'm sure many of you folks have used wide, fast lenses remotely where a full-size display is not practical. What do you do to make sure you haven't wasted a clear night by shooting a bunch of out-of-focus light frames?
Here's the quick processing, warts and all.