Tuning Knobs for the NLC Final Focus
  Compensation of optics errors at the Interaction Point (IP) is essential for maintaining maximum luminosity at the NLC. Several correction systems (knobs) using the Final Focus sextupoles have been designed to provide orthogonal compensation of linear and the second order optics aberrations at IP. Tuning effects of these knobs on the 250 GeV beam were verified using tracking simulations. 
Y. Nosochkov, P. Raimondi, T.O. Raubenheimer, A. Seryi, M. Woodley