The Fosic Resonator is an item added by Astral Sorcery. It is used to find locations of high concentrations of starlight to help power the player's Starlight Altar. When holding this device in a hand, areas of high starlight will have a blue mist over them at night. Higher concentrations have white sparkles coming off of the block. These locations are great for Altars and Spectral Relays.
