Moment tensor rate functions from waveforms with non-homogeneous variance

When discussing error estimates of the point-source mechanism and the source time function obtained by the two-step procedure by Šílený, Panza & Campus (1992), the authors insist that in the first step—inversion of seismograms (after Sipkin 1982) to get the moment tensor rate functions (MTRFs)—a homogeneous variance for all the data is needed to keep the advantageous symmetry of the normal equations. We show that this is too strong a requirement and can be dropped.