Pulsar lensing geometry
  Our analysis of archival VLBI data of PSR 0834+06 revealed that its
scintillation properties can be precisely modelled using the inclined sheet
model (Pen & Levin 2014), resulting in two distinct lens planes. These data
strongly favour the grazing sheet model over turbulence as the primary source
of pulsar scattering. This model can reproduce the parameters of the observed
diffractive scintillation with an accuracy at the percent level. Comparison
with new VLBI proper motion results in a direct measure of the ionized ISM
screen transverse velocity. The results are consistent with ISM velocities
local to the PSR 0834+06 sight-line (through the Galaxy). The simple 1D
structure of the lenses opens up the possibility of using interstellar lenses
as precision probes for pulsar lens mapping, precision transverse motions in
the ISM, and new opportunities for removing scattering to improve pulsar
timing. We describe the parameters and observables of this double screen
system. While relative screen distances can in principle be accurately
determined, a global conformal distance degeneracy exists that allows a
rescaling of the absolute distance scale. For PSR B0834+06, we present VLBI
astrometry results that provide (for the fist time) a direct measurement of the
distance of the pulsar. For targets where independent distance measurements are
not available, which are the cases for most of the recycled millisecond pulsars
that are the targets of precision timing observations, the degeneracy presented
in the lens modelling could be broken if the pulsar resides in a binary system.
