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That haze looks like it could be a camera light leak, through eyepiece and around the edge of the mirror --- since you're taking much longer exposures with bright light still hitting the outside of the camera, you're more sensitive to this. Have you tried covering up the viewfinder eyepiece during exposure? Many cameras come with a little rubber thingamajig on the strap designed to slip over the eyepiece, and recommend doing that for long exposures.
Thanks a lot for the tip, I will try covering the eyepiece next time though I don't have the rubber thingy... I will figure something out