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Strangely working flats
Hi APP folks,
As Ive had APP for quite some time IVe grown to stacking with the software. However ever since I upgraded to a newton (800mm f4 GSO + GPU coma corrector with canon 6d.) my flats don't work anymore. They either overcompensate the dust spots, or add a black bar. The latter is the most anoying because it adds a lot of gradients. I really don't know where to start from this point so Ive added some examples.
The first is the result
The second is actual data, you can see there's actual data in the lower bar.
Thanks in advance.
Oeh, that's pretty dramatic. Could you explain your exact workflow when taking flats?
And, how does the data look like when you don't use the flats and use light pollution correction?
Hello Vincent, thanks for your response.
1. My workflow taking flats is > Canon 6D in AV mode > Geoptik lightbox on top of my tube > Take 20-30 flats in 3 different stops (-1, 0 and +1). The -1 corrects the dustspots, the 0 and +1 stops overcompensate the dustspots.
2. The frame is filled with very anoying dust spots then and contrast loss, really a loss of immersion. I've added a unedited stacked frame where flats failed, and a equalised stacked frame to see the dust spots.
Ah ok, thanks for explaining! So, you are using the RAW data from the DSLR right? Also, I would check the histogram of the flats to determine the exposure. You don't want to be too low or too high, clipping the data. So when you load 1 flat-sub, you can see the histogram on the top-right. Go down to the drop-down menu for the stretch preset (standard it says 15%), and select "no stretch" there. Then you'll see the actual peak of the flat histogram as it was taken. That should be roughly in the middle, it's not very important to get it exactly there, it just has to be away from the extremes (either low or high). Can you check?
1. Yes I'm using raw image files.
2. I've checked the flats and indeed the -1 flats are off. Ive added a screenshot for elaboration. Actually all my flats are off, since i've got no way to make new flats (tube dissambly for maintenance), is there a way to get the histogram in the middle in the old flats?
Thanks in Advance.
Unfortunately not, the data is the way it is already. But looking at the peak, it should be fine actually, it doesn't have to be in the middle, it should be away from the left or right extremes.
So are you using proper bias or darkflats as well to calibrate the flats?
About the flats, its noted and I will test it next time I get a chance.
About the bias, im using some old bias frames I could actually make new ones and see if that works. And APP doesnt calibrate at all if you dont supply bias frames.