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We report the design of a miniature imaging spectrometer array (ISA) for observations of the daytime and nighttime mesosphere, capable of operating in a spectral range extending from the near-ultraviolet (NUV) to the near-infrared (NIR) -- 260 to 870 nm. The instrument comprises an array of f/2 all-reflective imaging spectrometers with a 6 degree(s) field of view. The design comprises an offset single aspheric toroidal telescope mirror, a slit, an offset aspheric toroidal collimator, a plane reflective grating and a camera with three offset decentered aspheric mirrors. The optical system has a 75 mm effective focal length and approximately 7.5 micrometers spot size. The slit image curvature distortion for the system is less than 7.5 micrometers . Sampling of the image plane is provided by a 1317 X 1035 spatial x spectral pixel CCD array with 6.8 micrometers X 6.8 micrometers pixel size. Three modules of the array cover the wavelength range 260 to 400 and 550 to 870 nm at 0.3 nm spectral resolution. One high resolution module covers the range 306 to 310 at 0.05 nm resolution. This channel is used for the measurement of the hydroxyl radical. The sensitivity in the mid visible is approximately 0.1 counts/R-s/spatial bin, dropping to approximately 0.05 count/R-s/bin in the NUV. The readout electronics software allows the 1317 spatial pixels to be summed into any number of selectable bin sizes incurring a single read per bin. Since much of the full slit sensitivity is attributable to the large (6 degree(s)) field of view, the slit could be slanted with respect to the vertical, in order to enhance the sensitivity per vertical spatial bin, at the cost of some horizontal smearing. The instrument offers a powerful means for conducting comprehensive spectroscopic studies of the lower thermosphere and mesosphere, since the overall performance is better than that of the Imaging Spectrometric Observatory (ISO) flown on the ATLAS 1 shuttle mission in 1992. The weight and size reduction from the ISO to the ISA are approximately 270 kg to < 15 kg, and 20 cubic feet to 1 cubic foot respectively. The instrument has been designed specifically to address the issue of quantifying the chemical reactions which result in the natural destruction of ozone in the upper atmosphere. Expected measurements include the concentrations of O, OH, O3 O2, N2, and the neutral temperature between 50 and 110 km. Concentrations H and HO2 are indirectly determined from the data. The design meets NASA Administrator Daniel Goldin's challenge to build better, cheaper, smaller instruments for fast turn around small satellite missions.
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