Optical validation and characterisation of Planck PSZ1 sources at the
  Canary Islands observatories. II. Second year of ITP13 observations
  We report new galaxy clusters previously unknown included in the PSZ1
catalogue. The results here presented have been achieved during the second year
of a 2-year observational programme, the ITP13, developed at the Roque de los
Muchachos Observatory (La Palma, Spain). We characterise 75 SZ sources with low
SZ significance, SZ S/N$<5.32$ by performing deep optical imaging and
spectroscopy in order to associate actual galaxy clusters to the SZ Planck
source. 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 SZ
sources with $Dec \geq -15^{\circ}$ (212 of them completely unknown), finding
optical counterparts for 152 SZ sources. The ITP13 validation programme has
allowed us to update the PSZ1 purity, which is now more refined, increasing
from 72\% to 83\% in the low SZ S/N regime. Our results are consistent with the
predicted purity curve for the full PSZ1 catalogue and with the expected
fraction of false detections caused by the non-Gaussian noise of foreground
signals. Indeed, we find a strong correlation between the number of unconfirmed
sources and the thermal emission of diffuse galactic dust at 857 GHz, thus
increasing the fraction of false Planck SZ detections at low galactic
latitudes.
