You can change the ENVI Preference under User Defined Files to point to your expression (.exp) file. Then when you launch ENVI, it should restore the .exp file in Band Math.

Is this an automated reply?

No, it is not an automated reply. It sounded like you were looking for a way to have an expression file automatically load when you launch ENVI. If this is not correct, could you please describe what you are trying to do?

I generate my expressions file. Here's an example: ENVI EXPRESSIONS byte ( (b1 eq 0)*0 or (b1 ne 0)* (1+254*((3.1415*( 0.9857 ^2)*( 12.733049918783493 +b1/ 1.1791480957373524 ))/(1032*cos((90- 46.54177889 )*(2*3.1415/360)))) )) byte ( (b2 eq 0)*0 or (b2 ne 0)* (1+254*((3.1415*( 0.9857 ^2)*( 15.557029800066955 +b2/ 1.9305274195606286 ))/(1537*cos((90- 46.54177889 )*(2*3.1415/360)))) )) byte ( (b3 eq 0)*0 or (b3 ne 0)* (1+254*((3.1415*( 0.9857 ^2)*( 28.977476474900893 +b3/ 2.1759233417561124 ))/(1808*cos((90- 46.54177889 )*(2*3.1415/360)))) )) Some of these parameters have been calculated and I want to document how that has been done. For example, to add a comment line, such as % sun elevation: calculated (lat=54.82476; lon=4.0802; GMT=9:09:38) Using the file as above is no problem at all, but I haven't been able to find an accepted syntax for comment lines.

Try adding a semicolon before the comment: ; percent sun elevation: calculated (lat=54.82476; lon=4.0802; GMT=9:09:38)