First results from an all-reflection spatial heterodyne spectrometer with broad spectral coverage

Operation of an all-reflection, broadband, spatial heterodyne spectrometer (SHS) is reported. This Mark 2 SHS is constructed using a custom diffraction grating and other standard optical components. The custom grating is coarse (18 grooves/mm), with a symmetric blaze that allows its simultaneous use as dispersing element and beam splitter and combiner. The grating is combined with a plane mirror and a roof mirror to form a very stable ring interferometer which has been used successfully in earlier narrowband SHS designs. Fringes from the extra grating orders in the main blaze envelope are unexpectedly found to combine constructively with the desired primary fringes of the interferometer. Elimination of ambiguity between wavelengths above and below blaze in a given order, and order separation are demonstrated using a small tilt of the plane mirror about an axis in the plane of the figure. Coverage of a factor of four in wavelength in a single CCD frame is demonstrated.