Optical spectroscopy of blazars for the Cherenkov Telescope Array

Context. Blazars are the most numerous class of high-energy (HE; E ∼ 50 MeV−100 GeV) and very (VHE; 100 GeV−10 TeV) gamma-ray emitters. Currently, a measured spectroscopic redshift is available for only about 50% BL Lacertae objects (BL Lacs), mainly due to difficulty in measuring reliable redshifts from their nearly featureless continuum-dominated optical spectra. The knowledge fundamental understanding emission blazars, population studies also indirect extragalactic background light searches Lorentz invariance violation axion-like particles using blazars. Aims. This paper first series papers that aim measure sample blazars likely be detected with upcoming Cherenkov Telescope Array (CTA), ground-based observatory. Methods. Monte Carlo simulations were performed select those hard spectrum Fermi -LAT telescope still lacking measurements, but by CTA 30 hours observing time or less. Optical campaigns involving deep imaging observations organised efficiently constrain redshifts. We medium- high-resolution spectroscopy 19 blazar counterparts Keck II, SALT, ESO NTT telescopes. searched systematically spectral features and, when possible, we estimated contribution host galaxy total flux. Results. eleven firm values ranging 0.1116 0.482, one tentative redshift, three lower limits including at z ≥ 0.449 another 0.868. Four Lacs show