The correct scale factor will depend on what type of ASTER data product you have, and what units are used for the pixel values in the thermal bands. You may need to refer to the data provider for this information. I believe that ASTER level 1a data are not yet calibrated to radiance, and ASTER level 2b data are. Before running the Thermal Atmospheric Calibration tool on your ASTER thermal bands, you will want to make sure that the pixel values are in radiance. If you need to calibrate them to radiance, this can be done using ENVI's Radiometric Calibration tool. In that tool, if you *do not* set a non-default scale factor or click the button called "Apply FLAASH Settings", then the output radiance thermal images will use units of W/(m2 * sr * µm). In this case, you can use a scale factor of 1.0 in the Thermal Atmospheric Correction tool, because the pixel values are already in the expected radiance units. If the pixel values in your ASTER image are already in radiance, then you will need to find out what units are used. I think that ASTER Level 1b data products are usually distributed with thermal bands already in units of W/(m2 * sr * µm). But you will want to double check that with your data provider. If the radiance units are not W/(m2 * sr * µm), then you will want to enter a scale factor in the Thermal Atmospheric Correction tool that when multiplied by the original pixel values will convert them to W/(m2 * sr * µm). - Peg Exelis VIS