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PoGOLite is a balloon-borne hard X-ray polarimeter dedicated to the study ofpoint sources. Compton scattered events are registered using an array ofplastic scintillator units to determine the polarisation of incident X-rays inthe energy range 20 - 240 keV. In 2013, a near circumpolar balloon flight of 14days duration was completed after launch from Esrange, Sweden, resulting in ameasurement of the linear polarisation of the Crab emission. Building on theexperience gained from this Pathfinder flight, the polarimeter is beingmodified to improve performance for a second flight in 2016. Suchoptimisations, based on Geant4 Monte Carlo simulations, take into account thesource characteristics, the instrument response and the background environmentwhich is dominated by atmospheric neutrons. This paper describes theoptimisation of the polarimeter and details the associated increase inperformance. The resulting design, PoGO+, is expected to improve the MinimumDetectable Polarisation (MDP) for the Crab from 19.8% to 11.1% for a 5 dayflight. Assuming the same Crab polarisation fraction as measured during the2013 flight, this improvement in MDP will allow a 5{\sigma} constrained result.It will also allow the study of the nebula emission only (Crab off-pulse) andCygnus X-1 if in the hard state.

Optimising a balloon-borne polarimeter in the hard X-ray domain: from the PoGOLite Pathfinder to PoGO+