Error bars versus the peak S/N for different kinds of stars. Left top panel: the error bars on the null field for all stars calculated using both H Balmer and metal lines. Right top panel: same as left panel, but considering only main sequence A- and B-type stars (including Ap/Ap stars) observed using grism 600B, and using H Balmer lines only. Left bottom panel: same as right top panel, but using metal lines only. Right bottom panel: the errors σ_{⟨ Bz ⟩} on the longitudinal field measured from H Balmer lines only in white dwarfs; the outliers are the hottest stars.