This seems wrong... `b` is a loop variable but this line is not in the loop, meaning that `b` is a single band (the last one we looped over). I think this was `cwave` before. `cwave` is the 1-d array containing the bandpass effective wavelength for each observation. For example, for observations in `['desg', 'desg', 'desr', 'desr']`, `cwave = [4400., 4400., 5600., 5600.]` (or whatever the actual wavelengths are).