Stacking 4 nights of data was a nightmare, turns out my flats were the problem

I spent about 11 hours over 3 separate sessions trying to figure out why my final stacked image of M31 had these weird dark donut spots all over it. Everyone online kept saying it was calibration artifacts or my darks, so I kept re-shooting those. Ended up being my flat frames were overexposed, I was shooting them at like 1/2000s instead of matching the histogram. That simple fix took me nearly a week to stumble onto after reading a random forum comment buried on page 6. Has anyone else burned that much time on something that sounds so basic in hindsight?

2 comments

2 Comments

the_laura20d ago

Damn, how long did you have that histogram pegged before you caught it? I literally did the same thing with my Ha flats last year, spent 2 full weekends chasing dust motes that were actually just overexposed flat artifacts. The trick that finally clicked for me was checking the RGB values in the middle of the flat frame, making sure they sat around 30-40% of the max, not blown out to near white. After that I started shooting flats at the same ISO and roughly the same exposure time as my lights, just with a white t-shirt over the lens cap, and all those donut gremlins vanished. Sucks that the answer was hiding on page 6 of some random thread, but honestly that's like half of astrophotography troubleshooting lol.

aaronclark20d ago

Man, I felt that in my soul, I wasted a whole month on dust bunnies that were just my own bad flats lol.