Absolute luminosity against parallax error (σ_ϖ in Table A.1) of the RSG stars in χ Per cluster (black filled circles with error bars) compared with the standard errors of the photo-center displacements of the RHD simulations (red star symbol). For the calculation of the latter we use the standard deviations σ_P of Table 1 and transform to adopting the mean parallax of 0.442 mas of the blue star sample (see text).