This work is conducted in the context of the Redshift Determination Group of the CTA Consortium. It has made use of the CTA instrument response functions provided by the CTA Consortium and Observatory, see https://www.cta-observatory.org/science/cta-performance (version prod3bv1 and prod5v0.1) for more details. We acknowledge the use of data from the following collaborations or projets: Fermi-LAT, Pan-STARRS, SDSS, DSS, DES, 2MASS and Gaia.