  In recent times an increasing number of extended haloes and multiple shells
around planetary nebulae have been discovered. These faint extensions to the
main nebula trace the mass-loss history of the star, modified by the subsequent
evolution of the nebula. Integrated models predict that some haloes may be
recombining, and not in ionization equilibrium. But parameters such as the
ionization state and thus the contiguous excitation process are not well known.
The haloes are very extended, but faint in surface brightness - 10^3 times
below the main nebula. The observational limits lead to the need of an
extremely well studied main nebula, to model the processes in the shells and
haloes of one object. NGC2438 is a perfect candidate to explore the physical
characteristics of the halo. Long-slit spectroscopic data were obtained. These
data are supplemented by imaging data from the HST archive, and archival VLA
observations. The use of diagnostic diagrams draws limits for physical
properties in the models. CLOUDY is used to model the nebular properties, and
to derive a more accurate distance and ionized mass. We derive an accurate
extinction E(B-V)=0.16, and distance of 1.9kpc. This puts the nebula behind the
nearby open cluster M46. The low-excitation species are found to be dominated
by clumps. The emission line ratios show no evidence for shocks. We find the
shell in ionization equilibrium: a significant amount of UV radiation
infiltrates the inner nebula. Thus the shell still seems to be ionized. The
spatially resolved CLOUDY model supports the hypothesis that photoionization is
the dominant process in this nebula, far out into the shell. Previous models
predicted that the shell would be recombining, but this is not confirmed by the
data. We note that these models used a smaller distance, and therefore
different input parameters, than derived by us.
