of $i=19.1$, but since high-redshift quasars are rare, objects lying in regions of color-space corresponding to quasars at $z\gtrsim3$ are targeted to $i=20.2$ (PSF magnitudes corrected for Galactic extinction based on the dust maps of Schlegel, Finkbeiner & Davis 1998).

### SDSS-III BOSS DR9 Quasars

The Data Release 9 Quasar catalogue (DR9Q; P[â]{}ris et al. 2012) is the first quasar catalogue from the Baryon Oscillation Spectroscopic Survey (BOSS; Dawson et al. 2012) of the Sloan Digital Sky Survey III (Eisenstein et al. 2011). This catalogue contains 87,822 quasars over 3,275 deg$^2$. The primary science goal for the high-$z$ BOSS quasar survey was to measure the baryon acoustic oscillations in the Lyman-$\alpha$ forest (Busca et al. 2013; Slosar et al. 2013; Delabuc et al. 2014). As such a priority was set on obtaining a high-surface density of $z>2.1$ quasars in order to maximise the number of Lyman-$\alpha$ forest lines of sight observable in the blue. To achieve this high-surface density, quasar candidates were selected in a heterogenous manner, down to a magnitude limit of $g\leq22.0$ or $r\leq21.85$ (Ross et al. 2012). Quasars were selected based on their optical fluxes and colours, as well as their near-infrared, ultraviolet fluxes and radio properties; 