1/50 might be a little low... This is from the website I shared: "Your flat panel might be too bright and sky might be too bright which will cause your ISO1600 flats to be of very, very short exposure, short as fraction of a second. That will lead to a shutter shadowing effect (sensor will record shutter movement) and cause to non-uniform illumination across the chip. You should aim for at least 0.5 second exposure to eliminate this effect, better off, if you can do 1-2 seconds exposures for flat and not slower."
I use my iPad, and I had to decrease the brightness significantly so I could do around 2 - 3 seconds, which works very well for me. What happened to me, and what the website explains, is that when my brightness is low enough to work at 2-3 seconds in AV mode on my
DSLR, it exposes perfectly at half-way in the camera histogram, but still not where you need it to be in
PI, which is showing a mean ADU of only 3500.
First thing I did was take an overexposed flat at 15 seconds at maximum brightness, and loaded the image in
PI to look at the maximum mean ADU value (where it is equal to the median). I measured my own maximum mean ADU and I saw that my max mean value was around 16000, so I wanted my flats mean ADU to be half of that around 8000. So what I did was (and this is where the website came in), while in AV mode, I increased the "exposure composition" to above 2, because I needed my ADU to be a little above twice as large as it was.
So you're exposing at around 4,500 mean ADU, and lets say you need the ADU to be around 8,000 like mine (for example), you would increase the exposure composition while in AV mode to 2. But you need to find out what your Max mean ADU is, first.
This is why Stu told you to post a flat at 10x the exposure (I think). This will allow us to look at the maximum mean ADU value to tell you what you SHOULD be exposing at.I don't see Pixinsight under your software, and I don't know if you have the ability to look at ADU with other software, so if you want to take an over exposed flat at the maximum level brightness with "exposure composition" at the default of 0, you can post it here and we can tell you what your ADU should be.
I hope this makes sense! I just spent a month figuring all this out myself haha
The website has pictures to, which might help with explaining all of that ^