ResoNique is a specialized optical calculator that lets you instantly determine the free spectral range, finesse, and filter bandwidth of Fabry–Pérot etalons with simple input parameters such as cavity length, refractive index, and mirror reflectivity. Built with precision and a modern, colorful interface, it is designed for researchers, optical engineers, and students working in spectroscopy, telecommunications, and photonics, providing fast, accurate, and user-friendly results that save time in experimental design and analysis.