Modal Noise in High‐Resolution, Fiber‐fed Spectra: A Study and Simple Cure

The transmission modes of a fiber introduce an unstable intensity distribution in high-resolution spectra which cannot be eliminated by flat-fielding because the pattern is sensitive to any small changes in fiber position or illumination. These instabilities, rather than photon shot noise, limit the achievable signal-to-noise ratio (S/N). In controlled laboratory experiments at a resolution of ~150,000, we found S/N limited to ≤500 for a range of fiber perturbations and illumination geometries from simple displacement to simulated seeing variations. We find an empirical relation between the number of monochromatic modes and the level of modal noise for which we offer no rigorous explanation. More important, we demonstrate that continuous, low-amplitude agitation of the fiber close to its output completely eliminates the problem.