  The level of electronic correlation has been one of the key questions in
understanding the nature of superconductivity. Among the iron-based
superconductors, the iron chalcogenide family exhibits the strongest electron
correlations. To gauge the correlation strength, we performed systematic
angle-resolved photoemission spectroscopy study on the iron chalcogenide series
Fe$_{1+y}$Se$_x$Te$_{1-x}$ (0$<$x$<$0.59), a model system with the simplest
structure. Our measurement reveals an incoherent to coherent crossover in the
electronic structure as the selenium ratio increases and the system evolves
from the weakly localized to more itinerant state. Furthermore, we found that
the effective mass of bands dominated by the d$_{xy}$ orbital character
significantly decreases with increasing selenium ratio, as compared to the
d$_{xz}$/d$_{yz}$ orbital-dominated bands. The orbital dependent change in the
correlation level agrees with theoretical calculations on the band structure
renormalization, and may help to understand the onset of superconductivity in
Fe$_{1+y}$Se$_x$Te$_{1-x}$.
