In helical antiferromagnets such as dysprosium, the magnetic structure is an helix incommensurate with the crystal lattice. Domains are therefore defined not only by the chirality of the magnetic helix, but also by its phase. While domains of the former can be evidenced using incoherent X-ray beams, coherence is necessary to evidence the latter. The situation is further complicated in diffraction by the overlap of charge and magnetic signals, which have different polarisations. We propose that Bragg ptychography, with a multimodal phase retrieval algorithm, can solve this difficulty and allow imaging such helical phase domains for the first time. Moreover, the measurements will provide simultaneously a microscopic view of the crystal structure. The relationship between heterogeneities of the crystal structure and the magnetic structure will provide fundamental insight about the phase degree of freedom of the magnetic structure.