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When you have ASTER Image lever 3b, you data is in Radiance you need to convert that data in Reflectance, with ENVI with FLASSH module you will use the make atmospheric corrections to change radiance values to reflectance to do spectral analyst, and another important thing is resampling your spectral library that you can use with ASTER. I can no make the exact process, because I don't know all the steps. This is a bit of what I know.