High-resolution broadband spectroscopy using an externally dispersed interferometer

An externally dispersed interferometer (EDI) is a series combination of a fixed delay interferometer and an external grating spectrograph. We describe how the EDI can boost the effective resolving power of an echelle or linear grating spectrograph by a factor of 2 - 3 or more over the spectrograph's full bandwidth. The interferometer produces spectral fringes over the entire spectrograph's bandwidth. The fringes heterodyne with spectral features to provide a low spatial frequency moire pattern. The heterodyning is numerically reversed to recover highly detailed spectral information unattainable by the spectrograph alone. We demonstrate resolution boosting for stellar and solar measurements of two-dimensional echelle and linear grating spectra. An effective spectral resolution of similar to 100,000 has been obtained from the similar to 50,000 resolution Lick Observatory two-dimensional echelle spectrograph, and that of similar to 50,000 from an similar to 20,000 resolution linear grating spectrograph.