We present advances in experimental techniques of apertureless scanning near-field optical microscopy (aSNOM). The rational alignment procedure we outline is based upon a phase singularity that occurs while scanning polarizers around the nominal cross-polarized configuration of s-polarized excitation and p-polarized detection. We discuss the theoretical origin of this topological feature of the setup, which is robust against small deviations, such as minor tip misalignment or shape variations. Setting the polarizers to this singular configuration point eliminates all background signal, allowing for reproducible plasmonic eigenmode mapping with optimal signal-to-noise ratio. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3693346]