The code below works for one equation using math_doit. How could I do multiple equations on multiple images in a folder. Fow example the below code calculates NBR from (b9-b24)/(b9+b24) and outputs a filename with filename_NBR. How could I also get NDVI from (b9-b5)/(b9-b5) and have it output as filename_NDVI. Thanks   pro bandmath_NBR compile_opt idl2 input_path = 'E:\FinalProcessing\GrassValley_Slide\georef_atcoroutput\' cd, 'E:\FinalProcessing\GrassValley_Slide\burn_indices\'     files=FILE_SEARCH(input_path+'*_atm.bsq_georef', count=count)     IF(count EQ 0)THEN BEGIN   PrintF, 'No files were found to process' ENDIF ;open file     for i=0, count-1 do begin       ENVI_OPEN_FILE, files[i], r_fid=fid       if(fid eq -1) then begin         envi_batch_exit         return     endif envi_file_query, fid, dims=dims, ns=ns, nl=nl, fname=filename ;set pos array for calculation EVI t_fid=[fid,fid] pos = [9, 24]-1 exp = '(b9-b24)/(b9+b24)' ;set the output filenames by stripping out the base name and appending 'nbr.img' out_name = file_basename(filename, '_atm.bsq_georef')+'_NBR' ;call the doit envi_doit,'math_doit',$     fid=t_fid, pos=pos, dims=dims,$     exp=exp, out_name=out_name, $     r_fid=r_fid print, 'done'  endfor end