ALMA high performance nutating subreflector

For the international ALMA project's prototype antennas, we have developed a high performance, reactionless nutating subreflector (chopping secondary mirror). This single axis mechanism can switch the antenna's optical axis by ±1.5′ within 10 ms or ±5′ within 20 ms and maintains pointing stability within the antenna's 0.6″ error budget. The light weight 75 cm diameter subreflector is made of carbon fiber composite to achieve a low moment of inertia, < 0.25 kg m². Its reflecting surface was formed in a compression mold. Carbon fiber is also used together with Invar in the supporting structure for thermal stability. Both the subreflector and the moving coil motors are mounted on flex pivots and the motor magnets counter rotate to absorb the nutation reaction force. Auxiliary motors provide active damping of external disturbances, such as wind gusts. Non contacting optical sensors measure the positions of the subreflector and the motor rocker. The principle mechanical resonance around 20 Hz is compensated with a digital PID servo loop that provides a closed loop bandwidth near 100 Hz. Shaped transitions are used to avoid overstressing mechanical links.