 Order-safe angular scanning of a Fabry–Pérot spectrum

Spectral resolution and order identification are separate design constraints. An instrumental width narrower than a line separation does not prevent a different interference order from placing another wavelength at the same angle. Choose optical thickness so the source range fits a known order interval, sufficient finesse to resolve neighbouring lines, and an angular window or order-sorting filter to exclude unwanted orders. Material dispersion and the internal/external angle conversion must be calibrated.