Imaging spectrometers, also referred to in some contexts as hyperspectral imagers, have found a wide range of applications, from the laboratory to industrial process control and to remote sensing and astronomy. Such instruments may be thought of either as extending the capabilities of two-dimensional imaging by adding a third—spectral—dimension or as extending spectroscopy by collecting spectra from a large number of points arranged in a spatial context. Dispersive spectrometers, which use either a prism or a grating to divert light as a function of wavelength, are a popular design basis for imaging spectrometers. In typical modern embodiments, light enters the spectrometer through a slit, after which it is collimated by an optical system before encountering a prism or grating and another optical system (or in some forms the same element used to collimate) that forms a spectrally dispersed image of the slit on a two-dimensional detector array. Thus one dimension on the detector array (perpendicular to the entrance slit) corresponds to wavelength while the other corresponds to spatial position along the slit. Front optics are often used to image the object of interest onto the slit and some mechanism or motion is then used to build up the second spatial dimension of the data set by acquiring successive exposures as the slit is scanned across the object (or vice versa). In the field of remote sensing such a device is often referred to as a “pushbroom” imaging spectrometer, with the scan motion typically provided by the platform (aircraft or spacecraft) carrying the instrument.
Imaging spectroscopy places additional demands on the design of the optics beyond the requirements of a nonimaging spectrometer, since the image quality in the direction parallel to the slit is now an important factor, a challenge that the authors address in a novel approach, adopting Schwarzschild optical systems to obtain designs that are anastigmatic at the center of the slit and for a particular wavelength. The significance of the work is admirably placed in context by the authors in their discussion of how one would modify each of four other design systems—a Czerny–Turner spectrometer, an Ebert–Fastie spectrometer, an Offner spectrometer, and a Schwarzschild imager—to obtain their Schwarzschild spectrometer. The Offner in particular is a powerful design for imaging spectrometers, one from which the authors’ approach differs by the replacement of the Offner’s convex grating by a planar grating and a convex mirror. A cited advantage of using a planar rather than convex grating is flexibility, in that the spectral range may now be tuned (shifted) by rotation of the grating. Another significant distinction is that manufacture of high-efficiency gratings on curved surfaces is currently an expensive process, perhaps making the Schwarzschild design a particularly attractive option for low-cost or high-production applications.
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