It has two coaligned X-ray optics which focus X-ray photons onto two independent shielded focal plane modules (FPMs), namely FPMA and FPMB. Thanks to its focusing optics, it has a broad and high quality spectral coverage from 3 to 79 keV, a field of view (FoV) at 10 keV of 10  $\times$ 10  and an 18  FWHM with a half-power diameter of 58  [@2013ApJ...770..103H]. Given these features, *NuSTAR* is suitable for studying the hard X-ray spectra of AGN with high sensitivity, discriminating between the transmitted nuclear emission (i.e. radiation which penetrates the obscuring matter along the line of sight) and the scattered or reflected component (i.e. radiation which interacts with circumnuclear gas and gets absorbed or Compton scattered). One of the *NuSTAR* scientific goals is to study the Compton-thick ($N_{\rm H} > 1.5 \times 10^{24}$ cm$^{-2}$) AGN population, which is still poorly understood due to the lack of good quality spectra above 10 keV. This class of obscured active nuclei (see [@2004mas..conf..323C], for a review) is predicted from population synthesis models of the X-ray background to be a non-negligible contributor to the $\sim$ 30 keV peak of the cosmic X-ray background (CXB), which is still today mostly unresolved ([@2015ApJ...808..185C; @2015ApJ...808..184M]; 