def METHOD_NAME(self, ispec, wavelength):
    """
    Return CCD pixel width in Angstroms for spectrum ispec at given
    wavlength(s).  Wavelength may be scalar or array.
    """
    ww = self.wavelength(ispec, y=np.arange(self.npix_y))
    dw = np.gradient( ww )
    return np.interp(wavelength, ww, dw)