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tokens: 1966 ) and the effect on measured image quality was seen by the smaller FWHM of star images at redder wavelengths compared to 4900 Å in the MUSE cubes. Given that the wave feature of the extinction maps occurs where the surface brightness decreases by a factor 2 over a distance of a few spaxels (0.5), then the effect of the different image FWHM between the wavelengths of the H Balmer lines would modulate the ratio image which forms the basis of the extinction determination. For example for a step function across the shell, the ratio map H/H would be expected to show a trough, since the image FWHM at H is lower than at H. On the 10 s MUSE cube the measured FWHM of the central star varies from 0.53 to 0.50 between H and H. In order to determine if this effect could be responsible for the observed wave-trough appearance across the inner shell, the H image was convolved with a Gaussian to the same measured FWHM as the H image and the extinction map redetermined. The strength of the trough is slightly decreased, by 0.02 in , and the wave increased by a lesser amount, but the features remain. A repeat of this test but with the 120 s H map showed a similar result. Having rejected instrumental or seeing effects as causing the pattern in extinction, astrophysical effects are now examined. 3.4. Balmer v. Paschen line ratios The He II Pickering series 6g-4f 6560.10 Å line is within 2.7 Å of the H line and at the resolution of the MUSE spectra, and given the very large flux ratio (100), will be included in the flux of the fitted H line. Thus the H flux needs to be corrected for the contamination of the He II line (1% of the H surface brightness in the bright He II core of the nebula). The He II Pickering 7g-4f 5411.52 Å line was measured in all the cubes and from the Case B ratio 6560.1/5411.5 Å ratio, the 6560.1 Å observed line flux was computed using Storey Hummer (1995) and a reddening map. The observed value of the He II 6560.10 Å line of 0.0338 F(H) tabulated by Fang Liu (2011) appears to be too large (the theoretical ratio 6560.1/5411.5 Å is 1.75 at 10 K whilst Fang Liu 2011 have a dereddened ratio of 2.92). Similarly the Pickering He II 8g-4f 4859.32 Å line is blended with H at the MUSE resolution, so its contamination of the measured H flux should also be corrected (similarly 1% of H in the core). The theoretical ratio of 4859.3 Å to 5411.5 Å was again used. Applying both these corrections to the H and H flux maps, the derived extinction map was re-computed. The He II line is indeed only strong inside the inner rim and is very weakly present outside the rim, so the effect of the correction to H and H fluxes for He II is insignificant where the strongest features of the extinction are present. Inside the inner rim, where the He II is brightest, the effect on the extinction map is small, . The Paschen lines higher than P9 (9229.0 Å) are also available for extinction determination. These lines also have the advantage that the contaminating He II