We present an overview of the "KIFF" project, which provides ultra-deep K s-band imaging of all six of the Hubble Frontier Fields clusters, Abell 2744, MACS-0416, Abell S1063, Abell 370, MACS-0717, and MACS-1149. All of these fields have recently been observed with large allocations of Directors' Discretionary Time with the Hubble and Spitzer telescopes, covering and 3.6-4.5 , respectively. VLT/HAWK-I integrations of the first four fields reach 5σ limiting depths of (AB, point sources) and have excellent image quality (FWHM ∼ 0.″4). The MACS-0717 and MACS-1149 fields are observable from the northern hemisphere, and shorter Keck/MOSFIRE integrations on those fields reach limiting depths of K s = 25.5 and 25.1, with a seeing FWHM of ∼ 0.″4 and . In all cases the K s-band mosaics cover the primary cluster and parallel HST/ACS+WFC3 fields. The total area of the K s-band coverage is 490 arcmin2. The K s-band at 2.2 crucially fills the gap between the reddest HST filter (1.6 band) and the IRAC 3.6 passband. While reaching the full depths of the space-based imaging is not currently feasible from the ground, the deep K s-band images provide important constraints on both the redshifts and the stellar population properties of galaxies extending well below the characteristic stellar mass across most of the age of the universe, down to and including the redshifts of the targeted galaxy clusters (). Reduced, aligned mosaics of all six survey fields are provided.
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