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(Abridged) The HST Frontier Fields project started at the end of 2013 withthe aim of providing extremely deep images of 6 massive galaxy clusters. One ofthe main goals of this program is to push several telescopes to their limits inorder to provide the best current view of the earliest stages of the Universe.We present a detailed analysis of $z$$\sim$8 objects behind the HFFs lensingcluster, MACS0416-2403, combining 0.3-1.6 $\mu$m imaging from HST, ground-based$K_s$ imaging from VLT HAWK-I, and 3.6 $\mu$m and 4.5 $\mu$m Spitzer SpaceTelescope. We apply the classical Lyman Break technique, combiningnon-detection criteria and color-selection. In order to avoid contamination bymid-$z$ interlopers, we require a strong break between optical and nearinfrared data. The luminosity function at $z$$\sim$8 is computed using a MCmethod taking advantage of the SED-fitting results. A piece of cautionaryinformation is gleaned from new deep optical photometry of a previouslyidentified $z$$\sim$8 galaxy in this cluster, which is now firmly detected as amid-$z$ interloper with a strong $\approx1.5$ mag Balmer break (between F606Wand F125W). Using the SED of this interloper, we estimate the contaminationrate of our MACS0416$-$2403 sample, as well as for previous samples in Abell2744 based on HFF data. Our selection recovers 4 robust objects withm$_{F160W}$ ranging from 26.0 to 27.9 AB and located in modest amplificationregions ($\mu<$2.4). Two of the objects display a secondary break between theIRAC 3.6 $\mu$m and 4.5 $\mu$m bands which could be associated to the Balmerbreak or emission lines at $z$$\sim$8. The candidates generally have starformation rates around $\sim$10 M$_{\odot}$/yr and sizes ranging from 0.2 to0.5 kpc, in good agreement with previous observations and expectations forobjects in the early Universe.

Frontier Fields : Combining HST, VLT and Spitzer data to explore the $z$$\sim$8 Universe behind the lensing cluster MACS0416$-$2403