Imaging spatial heterodyne spectroscopy: theory and practice

Abstract

Spatial heterodyne spectroscopy (SHS) is a Fourier transform spectroscopy technique capable of very high spectral resolution with no moving parts. The original work described non-imaging SHS, and it became a true imaging spectroscopy technique without much fanfare. We describe the imaging technique and relate it to another 'image plane interferometer' described by Horton, called HEIFTS, which does not have heterodyned fringes. One configuration of a basic SHS is a two-beam Michelson interferometer with both mirrors replace by diffraction gratings operated in the Littrow configuration. To convert a parallel light SHS device to imaging, incoming light must be focused on the gratings, so that each grating receives a full image of the scene. To acquire 3D data cubes, the open field of view is scanned across the scene. Although the focal plane array sees both spatial and spectral information, they are not mixed, because information is encoded by optical path difference according to position on the focal plane.