  Blazars are active galactic nuclei, and the most numerous High Energy (HE)
and Very High Energy (VHE) gamma-ray emitters. Their optical emission is often
dominated by non-thermal, and, in the case of BL Lacs, featureless continuum
radiation. This renders the determination of their redshift extremely
difficult. Indeed, as of today only about 50 % of gamma-ray blazars have a
measured spectroscopic redshift. The knowledge of redshift is fundamental
because it allows the precise modeling of the VHE emission and also of its
interaction with the extragalactic background light (EBL). The beginning of the
Cherenkov Telescope Array (CTA) operations in the near future will allow the
detection of several hundreds of new BL Lacs. Using the first Fermi catalogue
of sources above 10 GeV (1FHL), we performed simulations which demonstrate that
at least half of the 1FHL BL Lacs detectable by CTA will not have a measured
redshift. Indeed the organization of observing campaigns to measure the
redshift of these blazars has been recognized as a necessary support for the
AGN Key Science Project of CTA. Taking advantage of the recent success of an
X-shooter GTO observing campaign, we thus devised an observing campaign to
measure the redshifts of as many as possible of these candidates. The main
characteristic of this campaign with respect to previous ones will be the use
of higher resolution spectrographs and of 8 meter class telescopes. We are
starting submitting proposals for these observations. In this paper we will
briefly describe the selection of the candidates, the characteristics of these
observation and the expected results.
