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We performed a detailed photometric analysis and photoionisation modelling onthree high excitation multiple shell planetary nebulae: NGC 3242, NGC 6826 andNGC 7662. Archival HST/WFPC2 narrow band filter images were used to investigateshocked regions by two independent methods: using low excitation ions (via Halpha/[N II] vs H alpha/[S II] extended diagnostic diagrams) and by means ofhigh excitation species looking for regions of enhanced [O III]/H alpha lineratios. Shocked region analysis via low excitation ions shows that majordeviations from the general inside to outside ionisation trend correspond onlyto regions where FLIERs or LISs are located. The reduction on the SNR at theoutskirts of the [O III]/H alpha ratio maps, made us unable to unambiguouslyidentified for an enhancement on the [O III]/H alpha line ratio as an indicatorfor shocks. For non-shocked regions we performed a photoionisation modellingusing Cloudy. Fittings to the [O III] and H alpha observed radial profiles leadus to constrain on the free parameters of the density laws and filling factorstogether to temperatures and luminosities for the CSPNe. Best fit models show avery well representation of the [O III] and H alpha emission. Discrepancies inthe model fittings to the [N II] and [S II] profiles at NGC 6826 and NGC 3242,can be attributed in the former case, due to a contamination by the light ofthe CSPN and, in the latter case, either due to gas inhomogeneities within theclumps or to a leaking of UV radiation.

HST/WFPC2 imaging analysis and Cloudy modelling of the multiple shell planetary nebulae NGC 3242, NGC 6826 and NGC 7662