In the example of Fig. 1 spectra formula_152 and formula_157 are the same, except that the range of the time-of-flight formula_158 differs. Panel a shows formula_159, panel b shows formula_160 and panel c shows their difference, which is formula_161 (note a change in the colour scale). Unfortunately, this map is overwhelmed by uninteresting, common-mode correlations induced by laser intensity fluctuating from shot to shot. To suppress such correlations the laser intensity formula_162 is recorded at every shot, put into formula_142 and formula_164 is calculated as panels d and e show. The suppression of the uninteresting correlations is, however, imperfect because there are other sources of common-mode fluctuations than the laser intensity and in principle all these sources should be monitored in vector formula_142. Yet in practice it is often sufficient to overcompensate the partial covariance correction as panel f shows, where interesting correlations of ion momenta are now clearly visible as straight lines centred on ionisation stages of atomic nitrogen.