  In this work, we propose a proper plasma analysis practice (PPAP), an updated
procedure of plasma diagnostics in the era of spatially-resolved spectroscopy.
In particular, we emphasize the importance of performing both of the extinction
correction and the direct method of plasma diagnostics simultaneously as an
integrated process. This approach is motivated by the reciprocal dependence
between critical parameters in these analyses, which can be resolved by
iteratively seeking a converged solution. The use of PPAP allows us to
eliminate unnecessary assumptions that prevent us from obtaining an exact
solution at each element of the spectral imaging data. Using a suite of
HST/WFC3 narrowband images of the planetary nebula, NGC 6720, we validate PPAP
by (1) simultaneously and self-consistently deriving the extinction, c(Hb), and
electron density/temperature distribution, (n_e, T_e), maps that are consistent
with each other, and (2) obtaining identical metal abundance distribution maps,
(n(N^+)/n(H^+), n(S^+)/n(H^+)), from multiple emission line maps at different
wavelengths/transition energies. We also determine that the derived c(Hb)
consists both of the ISM and circumsource components and that the ionized
gas-to-dust mass ratio in the main ring is at least 437 and as high as about
1600. We find that, unless we deliberately seek self-consistency, uncertainties
at tens of per cent can easily arise in outcomes, making it impossible to
discern actual spatial variations that occurs at the same level, defeating the
purpose of conducting spatially resolved spectroscopic observations.
