From the WISE All-Sky Explanatory Supplement, IV.4.h.i.1, and Jarrett+
        2011. These are relative response per erg and so can be integrated
        directly against F_nu spectra. Wavelengths are in micron,
        uncertainties are in parts per thousand.

def _load_data(self, band):
        """From the WISE All-Sky Explanatory Supplement, IV.4.h.i.1, and Jarrett+
        2011. These are relative response per erg and so can be integrated
        directly against F_nu spectra. Wavelengths are in micron,
        uncertainties are in parts per thousand.

        """
        # `band` should be 1, 2, 3, or 4.
        df = bandpass_data_frame('filter_wise_' + str(band) + '.dat', 'wlen resp uncert')
        df.wlen *= 1e4 # micron to Angstrom
        df.uncert *= df.resp / 1000. # parts per thou. to absolute values.
        lo, hi = self._filter_subsets[band]
        df = df[lo:hi] # clip zero parts of response.
        return df