Magnification process for BD +30°3639. In the left two columns, the images in [O iii] and [N ii] are shown. The third column shows the simple difference in corresponding [O iii] and [N ii] images from the two epochs (shown in the image headers). White-black difference patterns are the signatures of global growth between epochs. The residuals (fourth column) are similar to the difference images except that the earlier image was magnified by a factor M before the images were subtracted. The value of M is determined (by trial and error) to null the residual image as much as possible.