def METHOD_NAME(self,ispec=None,y=None):
    """
    returns wavelength evaluated at y
    """
    if y is None:
        y=np.arange(0,self.npix_y)
    else :
        y = np.atleast_1d(y)
    if ispec is None:
        ispec = np.arange(self.traceset.nspec)
    if np.size(ispec)==1 :
        return self.traceset.wave_vs_y(ispec,y)
    else :
        if np.size(y)==1 :
            res=np.zeros((ispec.size))
            for j,i in enumerate(ispec):
                res[j]=self.traceset.wave_vs_y(i,y)
            return res
        else :
            res=np.zeros((ispec.size,y.size))
            for j,i in enumerate(ispec):
                res[j]=self.traceset.wave_vs_y(i,y)
            return res