Simulated spectra of four BL Lacs using E-ELT equipped with an X-Shooter-like instrument (t_exp = 3600 s, R ~ 3000). For each object, we report two cases at z = 0.50 on the left panel and z = 1.00 on the right panel. The red solid line is the smoothed spectrum obtained by the adoption of a boxcar filter (9 pixels). The spectra represent the maximum N/H flux ratio for which it is possible to detect Ca II and G band absorption lines of the host galaxy (see details in Table 2).