def cart2polar(x, y, center=np.array([0, 0])):
    """
    transforms cartesian coords [x,y] into polar coords [r,phi] in the frame of the lense center

    :param coord: set of coordinates
    :type coord: array of size (n,2)
    :param center: rotation point
    :type center: array of size (2)
    :returns:  array of same size with coords [r,phi]
    :raises: AttributeError, KeyError
    """
    coordShift_x = x - center[0]
    coordShift_y = y - center[1]
    r = np.sqrt(coordShift_x**2+coordShift_y**2)
    phi = np.arctan2(coordShift_y, coordShift_x)
    return r, phi