def custom_to_radec(phi1,phi2,T=None,degree=False):
    """
    NAME:

        custom_to_radec

    PURPOSE:

       rotate a custom set of sky coordinates (phi1, phi2) to (ra, dec)
       given the rotation matrix T for (ra, dec) -> (phi1, phi2)

    INPUT:

        phi1 - custom sky coord

        phi2 - custom sky coord

        T - matrix defining the transformation (ra, dec) -> (phi1, phi2)

        degree - default: False. If True, phi1 and phi2 in degrees

    OUTPUT:

        (ra, dec) for vector inputs [:, 2]

    HISTORY:

        2018-10-23 - Written - Nathaniel (UofT)
    """
    if T is None: raise ValueError("Must set T= for custom_to_radec")
    return radec_to_custom(phi1, phi2,
                           T=nu.transpose(T),  # T.T = inv(T)
                           degree=degree)