Comparison between the NB and spectroscopic redshifts of the candidates. Top: retrieved Lyα redshift of the candidates with SDSS or HETDEX spectrum. The straight lines represent the redshift at which the most common QSO contaminant lines are selected as Lyα. Bottom: difference between the NB Lyα redshift of the good candidates and the spectroscopic redshifts given by SDSS or HETDEX. The error bars show the redshift interval covered by the width of the NB in which the candidate is selected.