def guess_DMstep(dt, BW, fctr):
    """Choose a reasonable DMstep by setting the maximum smearing across the
        'BW' to equal the sampling time 'dt'.
        
        Inputs:
            dt: sampling time (in seconds)
            BW: bandwidth (in MHz)
            fctr: centre frequency (in MHz)
    """
    return dt*0.0001205*fctr**3.0/(BW)