CRITICAL WARNING occurs only when including flats
Hi everyone, and thanks for reading this. Although I'm a seasoned veteran in astronomy, I'm pretty new to astrophotography. APP has helped a lot with my pursuits, for sure. For my latest session last night, I was using an 80mm f/7 refractor with a Canon 60D DSLR. Messier 17, about 120 lights (30s, 1600 ISO), 20 darks, 30 bias, and 30 flats. This was my first session doing flats (I know, I know...), which I did basically with the "white T-shirt method" and a white image with my iPad. Darks were the same exposure/ISO/temp as lights, bias used same ISO at 1/8000s, and flats were 1/6 second at ISO 1600.
In the Normalizing phase, APP popped up a CRITICAL WARNING (see below) which I don't completely understand given my process last night and many identical processes I've done recently that didn't include flats. After receiving the warning, I did exactly the same M17 process except that I didn't include any flats, and received no warning. I also did a similar process on another data set and received the warning (with flats), but I did not receive the warning when I left out the flats.
I'm sure that I'm just missing something, but I'm not clear what it is. Were my flat exposures too short (iPad panel too bright)? I've also attached the histogram from my camera for one of the flats. Any suggestions are appreciated - thanks!
This means that you have an issue in the calibration data. Very simply said, when using that data, APP sees that subtracting the noise from the lights causes pixels to go black (too much signal is removed). It then tries to inject noise to get it to a decent level, but even that doesn't work, meaning there is something going wrong with the calibration data for sure. It's indeed very good to try a few things like removing the flats, removing the darks etc. to see which data is causing it. Looking at the flats, you're showing a screenshot of your camera I think? These usually show non-linear histograms, in APP you can load in a flat and then go to the preview section on the right and select "no stretch", that should have the peaks roughly in the middle or 2/3 the way.
Hi Jon @macejk,
Were you able to find the source of the issue with your calibration data? Do we need to have a look to help out?
Hi Mabula, and thanks for checking in on me. Since I posted this, I have had better - although not perfect - luck with flats. Attached is a screenshot of the master flat histogram in APP (no stretch) for which I was having troubles when processing my M17 data. It seems a bit underexposed.
@vincent-mod Vincent, also thanks to you for pointing me in the right direction. I'm still experimenting with flats and am slowly progressing in my newbie trials.
Hi Kevin, @macejk
Okay, let us know if you need more assistance. Indeed, looking at the masterflat unstretched does indicate that you could have exposed a bit longer, that should increase the quality of the flats as well 😉