def METHOD_NAME(self,ispec=None,wavelength=None):
    """
    returns CCD y centroids for the spectra
    ispec can be None, scalar or a vector
    wavelength can be a vector but not allowing None #- similar as in specter.psf.PSF.y
    """
    if ispec is None :
        ispec = np.arange(self.traceset.nspec)
    else :
        ispec = np.atleast_1d(ispec)
    if wavelength is None :
        wavelength = self.wavelength(ispec)
    else :
        wavelength = np.atleast_1d(wavelength)
    if len(wavelength.shape)==2 :
        res=np.zeros(wavelength.shape)
        for j,i in enumerate(ispec):
            res[j]=self.traceset.y_vs_wave(ii,wavelength[i])
    else :
        res=np.zeros((ispec.size,wavelength.size))
        for j,i in enumerate(ispec):
            res[j]=self.traceset.y_vs_wave(i,wavelength)
    return res