Extinction factor c _Hβ is shown as derived from the observed Hα/Hβ ratio and assuming an intrinsic ratio of 2.87, which applies to a low density gas at T _e = 10,000 K. The X-axis lies along P.A. = 240° and the Y-axis along P.A. = 330°. The sample was 4'' wide and the data have been averaged along bins of 10 pixels, corresponding to 0 4. The distance shown is from the position of the central star. The data closest to that star have been edited out because of contamination, thus causing a central gap or in later similar images, artificial values near the central star.