How did the Frontier Fields campaign detect galaxies 100 times fainter than those in the HUDF?

By utilizing gravitational lenses, specifically massive galaxy clusters, to naturally magnify faint background light.

The Frontier Fields campaign represented the next evolution in maximizing faint object detection using Hubble, building upon the legacy of the Deep Fields. This campaign did not rely solely on increased observation time or a new telescope (initially); rather, it strategically utilized massive galaxy clusters as natural magnifying tools. These clusters, due to their immense gravity, warp spacetime, creating what are known as gravitational lenses. These lenses magnify the light from galaxies located far behind them, allowing astronomers to observe objects that would otherwise be too faint to detect, achieving sensitivity enhancements up to 100 times fainter than the already deep HUDF images.