def ShortenMask(mask, size):
    """
    

    Parameters
    ----------
    mask : tuple
        tuple of n 1d array of size l
        coordonate of l points
    size : int
        size of the shortened mask
        
    Returns
    -------
    shortened_mask : tuple
        tuple of n 1d array of size equal to the size parameter
        coordonate of "size parameter" points

    """
    shortened_mask = (mask[0][:size], mask[1][:size], mask[2][:size])
    
    return shortened_mask