IRISHS, the Infrared Imaging Spatial Heterodyne Spectrometer: a new pushbroom Fourier transform ultraspectral imager with no moving parts

Imaging spatial heterodyne spectroscopy (ISHS) was invented by Roesler and Harlander and applied to the far ultraviolet. It also has advantages for remote sensing applications in the visible and infrared bands. We have designed, assembled, and ground-tested a new instrument designed for eventual airborne or spaceborne deployment for imaging spectroscopy at ultraspectral resolution (1 cm(-1)). IRISHS is a true, imaging, Fourier transform spectrometer (FTS) with a fully open square field of view (no slit), where the third dimension required to assemble data cubes is acquired by scanning the FOV linearly over the scene. Spatially displayed fringes are differenced ("heterodyned") with a spatial frequency determined by a pair of diffraction gratings, giving easily sampled fringe patterns. The current system operates between 8 and 12.5 microns in six sub-bands and employs a 256x256 pixel focal plane array. In comparison to dispersive imaging spectrometers with equivalent resolution, ISHS has: 1) much larger etendue in a small package; 2) full-field imaging for ease in image reconstruction; 3) approximately the same signal-to-noise ratio (S/N) in equivalent observing scenarios, except in the case where the entire dispersive spectrometer is cryogenically cooled. In comparison to conventional FTS, ISHS has: I) no moving parts during data collection; 2) pushbroom imaging, 3) approximately the same SM for equivalent conditions, 4) much lower data rate.