  Polarimetric observations of celestial sources in the hard X-ray band stand
to provide new information on emission mechanisms and source geometries. PoGO+
is a Compton scattering polarimeter (20-150 keV) optimised for the observation
of the Crab (pulsar and wind nebula) and Cygnus X-1 (black hole binary), from a
stratospheric balloon-borne platform launched from the Esrange Space Centre in
summer 2016. Prior to flight, the response of the polarimeter has been studied
with polarised and unpolarised X-rays allowing a Geant4-based simulation model
to be validated. The expected modulation factor for Crab observations is found
to be $M_{\mathrm{Crab}}=(41.75\pm0.85)\%$, resulting in an expected Minimum
Detectable Polarisation (MDP) of $7.3\%$ for a 7 day flight. This will allow a
measurement of the Crab polarisation parameters with at least $5\sigma$
statistical significance assuming a polarisation fraction $\sim20\%$ $-$ a
significant improvement over the PoGOLite Pathfinder mission which flew in 2013
and from which the PoGO+ design is developed.

Instrumentation and Methods for Astrophysics