H$\alpha$ fluxes and extinction distances for planetary nebulae in the
  IPHAS survey of the Northern Galactic Plane
  We report H$\alpha$ filter photometry for 197 northern hemisphere planetary
nebulae (PNe) obtained using imaging data from the IPHAS survey. H$\alpha$+[N
II] fluxes were measured for 46 confirmed or possible PNe discovered by the
IPHAS survey and for 151 previously catalogued PNe that fell within the area of
the northern Galactic Plane surveyed by IPHAS. After correcting for [N II]
emission admitted by the IPHAS H$\alpha$ filter, the resulting H$\alpha$ fluxes
were combined with published radio free-free fluxes and H$\beta$ fluxes, in
order to estimate mean optical extinctions to 143 PNe using ratios involving
their integrated Balmer line fluxes and their extinction-free radio fluxes.
Distances to the PNe were then estimated using three different 3D interstellar
dust extinction mapping methods, including the IPHAS-based H-MEAD algorithm of
Sale (2014). These methods were used to plot dust extinction versus distance
relationships for the lines of sight to the PNe; the intercepts with the
derived dust optical extinctions allowed distances to the PNe to be inferred.
For 17 of the PNe in our sample reliable Gaia DR2 distances were available and
these have been compared with the distances derived using three different
extinction mapping algorithms as well as with distances from the nebular radius
vs. H$\alpha$ surface brightness relation of Frew et al. (2016). That relation
and the H-MEAD extinction mapping algorithm yielded the closest agreement with
the Gaia DR2 distances.
