Below is a three dimensional plot of the first raw image. The x and y plane coincide with the pixel numbers, and the pixel values are shown as a function of z. The z-range is between 1500 and 1600 which is where the values for the background are. The edges of the image are farthest away from the nebula, and so should give us good values for the background. Simply looking at the two exposed edges reveals that not only is the background not uniform, it suffers from two different gradients along both axes.
Similar situations exist for all of the raw images.
Simply subtracting a constant from the arrays is obviously not sufficient. We tried several different methods to minimize the gradient and get as even a background as possible, but with little success. The best results were obtained by attempting to individually scale down the aligned images until the background was near zero. Although the vectors appear more promising, the actual images are severely distorted. We also tried to subtract the background using and IDL procedure written by Professor Graham.