  We identify new clusters and characterize previously unknown Planck
Sunyaev-Zeldovich (SZ) sources from the first Planck catalogue of SZ sources
(PSZ1). The results presented here correspond to an optical follow-up
observational programme developed during approximately one year (2014) at Roque
de los Muchachos Observatory, using the 2.5m Isaac Newton telescope, the 3.5m
Telescopio Nazionale Galileo, the 4.2m William Herschel telescope and the 10.4m
Gran Telescopio Canarias. We characterize 115 new PSZ1 sources using deep
optical imaging and spectroscopy. We adopt robust criteria in order to
consolidate the SZ counterparts by analysing the optical richness, the 2D
galaxy distribution, and velocity dispersions of clusters. Confirmed
counterparts are considered to be validated if they are rich structures, well
aligned with the Planck PSZ1 coordinate and show relatively high velocity
dispersion. Following this classification, we confirm 53 clusters, which means
that 46% of this PSZ1 subsample has been validated and characterized with this
technique. Sixty-two SZ sources (54% of this PSZ1 subset) remain unconfirmed.
In addition, we find that the fraction of unconfirmed clusters close to the
galactic plane (at |b|<25deg) is greater than that at higher galactic latitudes
(|b|>25deg), which indicates contamination produced by radio emission of
galactic dust and gas clouds on these SZ detections. In fact, in the majority
of the cases, we detect important galactic cirrus in the optical images, mainly
in the SZ target located at low galactic latitudes, which supports this
hypothesis.
