ENVI has a multi-step process that can perform basic atmospheric correction then convert the resulting emissivity bands to a brightness-temperature image in degrees Kelvin. When ENVI reads an ASTER AST_L1B scene it calibrates the TIR bands to proper radiance values. If you are working with one of these datasets proceed to Step 2. For the newer ASTER AST_L1T datasets ENVI opens these with “byte values” which must first be converted to floating-point radiance values as shown in Step 1.
From the Toolbox select Radiometric Correction | Radiometric Calibration and select the five-band TIR file. Make sure the Calibration Type is Radiance, enter a new filename, and click OK
From the Toolbox select Radiometric Correction | Thermal Atmospheric Correction and select as input the five‑band TIR file for AST_L1B datasets or the file created in Step 1 for AST_L1T datasets. In the dialog window take all defaults and enter an output filename to create the input to the Emissivity Normalization process.
Again from the Toolbox, select Radiometric Correction | Emissivity Normalization and select the Thermal Correction file just created in Step 2. Take all defaults, make sure the Output Temperature Image is toggled to Yes and enter a filename for this.
You will now have a brightness temperature file with units in degrees Kelvin. You can convert this to Celsius using band math to subtract 273.15 from the file.