the image is a calibrated image with only histogram transform over applied so it becomes visible. I am aware there is a bit of drift due to flexure. I can get rid of the smeary background but I wonder if someone can give me a clue as to what it might be caused by. I thought it might be an older dark frame with uneven hot/cold pixels causing it but I just tried a brand new series of darks and the problem is still there. Is there any parameter in light calibration or image integration in PI that could cause this pattern rather than a random background?