Example DIA processing of the same original image (left), taken on the night of Sep. 20, and showing approximately 26′′, or 57 000 km, of the FORS image around the comet. The middle panel shows a DIA subtraction using the best seeing frame from the night of Sep. 20, and removes fainter stars well but demonstrates the negative imprint of the comet from its position in the template frame. The panel on the right uses a template image taken later in the year, but under poorer seeing conditions, resulting in noisier background subtraction and residuals left over from even faint stars, but without any negative comet image. Care was taken to choose the best template image for each night to maximise the number of frames where the comet was well separated from residual sources and any negative imprint.