OK.
Now try taking 19 or so of those ISO-400 dark frames and median combine them.
Don't stretch them or alter them in any way. Take them straight from the camera (ensure that the viewfinder is covered with the viewfinder cover which is attached to the strap when taking them) and median combine them in the astrophotographic processing software of your choice.
Post your resultant master dark frame -- no processing done on it.
H
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