The figure shows the basic optical configuration of an improved all-reflecting imaging spectrometer. This spectrometer would differ from older spectrometers in several respects, one being its convex spherical diffraction grating: heretofore, spectrometers have contained, variously, concave or plane diffraction gratings. The specific design, featuring the convex grating and two concave spherical mirrors, would afford a wide field of view and a compact layout that would make the size of this spectrometer about 1/3 that of a spectrometer of typical older design and similar capabilities. The sphericity of the optical surfaces would make fabrication relatively easy and inexpensive.

The Ray-Trace Diagram illustrates the basic function of the convex-grating spectrometer. In this view, the long dimension of the entrance slit and thus the spatial axis of the image plane lie perpendicular to the page, while the spectral dispersion axis of the image plane lies on the dotted line in the page.

The radii of curvature of the grating and mirrors and the off-axis positions and angles at which these optical components would be mounted have been selected to give the excellent spatial and spectral performance over the image plane. The spectrometer would produce a well-corrected, spectrally dispersed image of the entrance slit over a wide spectral range. The outstanding quality of the image would be attributable to good correction of astigmatism and field curvature over the image area. Heretofore, it has been extremely difficult to achieve correction of both field curvature and astigmatism over the image areas of imaging spectrometers.

The all-reflecting design would be suited for spectral regions from vacuum ultraviolet through visible to far infrared. Unlike in older imaging spectrometers, there would be no need for a field-flattening lens: this would constitute a major advantage for operation over extended spectral regions. The relative flatness of the focal plane would make this spectrometer suitable for use with a planar array of photodetectors.

The design would use the variation of astigmatism of the second mirror with wavelength to compensate for the variation of astigmatism of the grating with wavelength. The design would use the variation of astigmatism of both mirrors with field angle to compensate for the variation of astigmatism of the grating with field angle. The design would also use both spherical mirrors to compensate for the spectral and spatial field curvature intrinsic to the grating. Though the design does not look complicated, the process of optimizing the design was complicated because it involved careful selection of design parameters to balance astigmatism, field curvature, and coma over the spatial and spectral fields.

Design calculations show that in both the spectral and spatial aspects, the optical performance of this spectrometer would exceed that of any imaging spectrometer now in existence. Excellent spatial resolution perpendicular to the spectral dispersion axis would make this instrument attractive for a variety of industrial and medical applications that have already given rise to a multimillion-dollar market in imaging spectrometers.

This work was done by Michael P. Chrisp of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category, or circle no. 125on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
NASA Resident Office-JPL; (818) 354-5179.

Refer to NPO-19293.

This Brief includes a Technical Support Package (TSP).
Document cover
Convex-grating spectrometer with two spherical mirrors

(reference NPO19293) is currently available for download from the TSP library.

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