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We report new galaxy clusters previously unknown included in the PSZ1catalogue. The results here presented have been achieved during the second yearof a 2-year observational programme, the ITP13, developed at the Roque de losMuchachos Observatory (La Palma, Spain). We characterise 75 SZ sources with lowSZ significance, SZ S/N$<5.32$ by performing deep optical imaging andspectroscopy in order to associate actual galaxy clusters to the SZ Plancksource. We adopt robust criteria, based on the 2D-spatial distribution,richness and velocity dispersions to confirm actual optical counterparts up to$z<0.85$. At the end of the ITP13 observational programme, we study 256 SZsources with $Dec \geq -15^{\circ}$ (212 of them completely unknown), findingoptical counterparts for 152 SZ sources. The ITP13 validation programme hasallowed us to update the PSZ1 purity, which is now more refined, increasingfrom 72\% to 83\% in the low SZ S/N regime. Our results are consistent with thepredicted purity curve for the full PSZ1 catalogue and with the expectedfraction of false detections caused by the non-Gaussian noise of foregroundsignals. Indeed, we find a strong correlation between the number of unconfirmedsources and the thermal emission of diffuse galactic dust at 857 GHz, thusincreasing the fraction of false Planck SZ detections at low galacticlatitudes.

Optical validation and characterisation of Planck PSZ1 sources at the Canary Islands observatories. II. Second year of ITP13 observations