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tokens: accurately. Accounting for the large equivalent widths of the Hβ lines, we use a 600 s integration time for broad-line (i.e. narrow-band) observations. Figure 2. The SDSS-BOSS spectrum for Target-10. The transmission curve for the Hα filter used to measure the flux contained within the Hβ line is shown in orange and the SDSS i -band filter is shown in green. The region between 6820 and 6960 Å used to determine the median rest-frame L 5100 for the SDSS spectrum is shown in grey. Top: The whole spectrum. Bottom: The region between 6400 and 7000 Å which contains both the broad Hβ line and the region used to measure the rest-frame L 5100 . Our targets span a range of redshifts between 0.350 and 0.398. Therefore, for each source, we use the appropriate Hα photometric filter for which the redshifted Hβ line dominates. For Target-10, we use the Hα-6566 Å narrow-band filter. As seen in Table 1 , we obtain the largest number of acceptable exposures with SDSS-J144645.44+625304.0 (referred to as Target-10 hereafter). Indeed, Target-10 is the only QSO for which we have obtained a baseline of observations longer than the recommended 3 t H β (1 + z ) needed to recover a lag. Thus, in what follows, we only discuss the analysis of Target-10 and defer the rest to a future work. Table 1. Our pre-observation targets selected based on their luminosity, redshift, and visibility. t Hβ (1 + z ) is expected observed-frame lag derived from the Bentz et al. ( 2013 ) t rest – L 5100 relation, where L 5100 is estimated from the SDSS spectrum. We quote the length of time each target is visible and the baseline of the observations in units of days and expected observed-frame lag. 2.3 Ensemble photometry and flux calibration In order to estimate lags between the BLR and the continuum-emitting region of the QSO, we must first calibrate the i -band and Hα photometric magnitudes to a common magnitude system. We are then required to calibrate our i -band photometry using the known SDSS DR12 AB magnitudes of sources in the observed field. We calibrate Hα photometry by propagating available SDSS spectra through the transmission curve for the same narrow-band Hα filter (6566 Å) used to observe the Hβ line in Target-10, accounting for the fibre aperture. We perform aperture photometry using source extractor (Bertin Arnouts 1996 ) to estimate Petrosian magnitudes (Petrosian 1976 ; Graham et al. 2005 ) for each detected source in the field for both i -band and Hα exposures. We use Petrosian magnitudes in order to calibrate each exposure to the SDSS catalogue and to easily avoid the effects of differing seeing between our observations without modelling the point spread function (PSF). We consider only those sources which have SDSS clean = True and source extractor flags = 0 for use as reference sources. We can then apply a similar ensemble photometry method to that detailed by Honeycutt ( 1992 ), on the i -band exposures and calibrate those instrumental magnitudes to the SDSS