Sky subtraction for the edge-on galaxy UGC 01839 in the r_deep band of the IAC Stripe 82 Legacy Project data. Top: outer region of the very extended radial surface brightness profile of UGC 01839 (blue solid line). Overplotted is the same profile, but convolved with a one-dimensional Gaussian kernel of 1 pixel standard deviation (red solid line), the dashed lines represent the sky rms value (blue), the uncertainty of the whole background image (red), and the zero-count position (black); the mean sky value that we subtract from the image is shown with the cyan dotted line. Bottom: gradient function of the very extended surface brightness profile shown above; in this case, the flattest region of the gradient function was taken from 170 to 190 arcsec. UGC 01839 has an irregular background. However, there is a flat region between ∼170 and ∼190 arcsec that we use to obtain the mean sky value. Note that the sky had been oversubtracted in the original image.