Return the completeness as a function of magnitude.

        ADW: Eventually want a completeness mask to set overall efficiency.

def completeness(self, delta, method='step'):
        """
        Return the completeness as a function of magnitude.

        ADW: Eventually want a completeness mask to set overall efficiency.
        """
        delta = np.asarray(delta)
        if method == 'step':
            func = lambda delta: (delta > 0).astype(float)
        elif method == 'erf':
            # Trust the SDSS EDR???
            # 95% completeness: 
            def func(delta):
                # Efficiency at bright end (assumed to be 100%)
                e = 1.0
                # EDR says full width is ~0.5 mag
                width = 0.2 
                # This should be the halfway point in the curve
                return (e/2.0)*(1/np.sqrt(2*width))*(np.sqrt(2*width)-scipy.special.erf(-delta))
        elif method == 'flemming':
            # Functional form taken from Fleming et al. AJ 109, 1044 (1995)
            # http://adsabs.harvard.edu/abs/1995AJ....109.1044F
            # f = 1/2 [1 - alpha(V - Vlim)/sqrt(1 + alpha^2 (V - Vlim)^2)]
            # CAREFUL: This definition is for Vlim = 50% completeness
            def func(delta):
                alpha = 2.0
                return 0.5 * (1 - (alpha * delta)/np.sqrt(1+alpha**2 * delta**2))
        else:
            raise Exception('...')
        return func(delta)