I am trying to apply the MNF algorithm on MERIS L1B (no spectral subsetting). In the FORWARD TRANSFORM DIALOGUE box, I use SHIFT DIFF SUBSET button and isolate a region of pure pixels. I OK everything subsequently and try to run the MNF function. However I immediately get an error message which goes like this: mnf_doit: An error has occurred during processing Error: "Out of range subscript encountered: SPECTRA." I tried the process again without subsetting from the SHIFT DIFF SUBSET and the function worked but the results were hopelessly wrong, which means I need to subset pure some pure pixels first in order to work out the noise fractions. What am I doing wrong? What havent I considered?