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Chapter 17:
Some Trade-Offs
The best system for the aircraft example seemed to be the FTS. The best for the satellite seemed to be the pushbroom grating. The best for the simple Mars Rover demonstration was the filter wheel. Why is there no single best imaging spectrometer? What requirements push the solution in which direction? These are the general kinds of questions to be answered here. 17.1 General Considerations Of course, the significant parameters are the spectral range and resolution, the SNR for the application, the data rate, the size and efficiency of the optics and whole system, the size of the array, and the availability of all these components. 17.2 Optical Efficiency We can make a general comparison of efficiencies. Filters, depending on their resolving power, have a peak transmission of about 80%. Gratings have a blaze efficiency of about 60%. The double pass through a beamsplitter in an FTS requires that the transmission is less than 25%, more like 16% to 20%. 17.3 Bandwidth This consideration must be divided into two parts, two-dimensional arrays, and pushbroom, strip-mapper applications. For a CVF type of filter system, the frame is usually covered by a two-dimensional array. Therefore the bandwidth is determined by the frame rate and the number of spectral bins. So the filter approach is good when there are not too many spectral bins. The grating system spreads the spectrum over one dimension of the array. The bandwidth is not determined by the number of spectral bins, but by one direction of scan. The bandwidth is determined by the number of pixels in that one direction of scan. Thus, a grating is better when the size of the spatial frame is not too large and the angular subtense of a detector element not too small. The downside is that the more the spectral bins, the larger the array. The FTS system requires a given number of samples in a scan. It can be shown that the number of samples is just equal to the resolving power, the Q. The bandwidth is surely higher when the wavelengths are shorter and when the resolution is more demanding.
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