def METHOD_NAME(twth, wavelength, ang_units='degrees'):
    '''
    Converts 2th axis into q (q returned in inverse units of wavelength)
    q = [(4*PI)/lamda]*sin(2th/2)
    ang_unit : default in degrees; will convert from 'rad' if given
    '''
    if not ang_units.startswith('rad'):
        twth = DEG2RAD*twth
    return ((2*TAU)/wavelength)*sin(twth/2.)