In most cases, the final combined image will contain small but noticeable jumps in the vertical direction. This is caused by the imprecise removal of the bias when the dark was subtracted. The jumps can be removed very effectively by averaging the image along rows and subtracting the resulting one dimensional image from each column of the original image. To make sure that objects do not bias the result, one clips the 200 highest and lowest pixels from the computation of the one dimensional image.