Start with the first course if you don't know what the Wien approximation is. Try the second if you wonder about solar source functions. Study the third if you need detail on the Lambda operator or NLTE radiative equilibrium. The fourth is for IRIS spectrometrists and (optically) thin types. The fifth summarizes full-disk solar spectrum modeling. The last can be read as quick up-to-date overview or studied as comprehensive course encompassing the others.
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