Takaharu Ueda

Secondary mirror chopping system for large infrared telescope

A rapid back-and-forth motion of a secondary mirror is required for enhancement of weak signal-to-noise ratio by chopping the incoming beam to an infrared telescope. For a very large infrared telescope with a large secondary mirror (diameter of 500 mm, mass of 7 kg), a unique secondary mirror chopping system which is very light and suppresses mechanical vibration of a telescope during chopping motion is developed. The system has an active damper which cancels large reaction force to restrain excitement of the telescope. The damper is very compact and light, because a linear motor of the damper uses the same magnetic circuit as for a motor driving the mirror. The controller for moving the mirror and the damper consists of feedback and feedforward parts. A unique position command of the feedforward part accomplishes short transition time of the chopping motion without exciting resonances of the chopping mechanism. The system achieves a total mass of 100 kg, secondary mirror maximum chopping amplitude of 1.21 mrad, secondary mirror positioning accuracy of 4 microrad, and transition time of less than 10 msec without remaining vibration.

High speed and high precision servo system for large chopping secondary mirror

A high-speed and high-precision servo system is proposed for large chopping secondary mirror systems. It is also confirmed by experimental results that the servo system is very effective in realizing a high-performance chopping secondary mirror. The servo system has the following features: a mirror swung by a torque waveform which does not excite mechanical resonances, and realizes the desired positioning; a torque canceling feedback loop which uses the same stabilizing compensator as the mirror-swinging loop; and a two-degree-of-freedom control system, whose stability and speed of response are independently designed. From experimental results obtained from the test plant of the chopping secondary mirror of a 7.5 m infrared telescope, it is confirmed that the proposed servo is very effective in suppressing the remaining vibrations in the angular position of the mirror.<<ETX>>

Chopping secondary mirror system for 1.3-m IR telescope

In order to achieve a high speed/high precision/low power consumption IR telescope chopping system which excites no mechanical resonances, in conjunction with a device size smaller than the secondary mirror diameter, a chopping servo has been developed which uses feedforward control with optimized driving torque as well as feedback control guided by robust stability theory. The compactness of the device has been achieved through the use of an Nd-Fe-B magnet motor and the balancing of the mirror against the mass of the electromechanical system. The chopper has a 10-msec transient time, and stability in excess of 1 arcsec, for square-wave chopping with amplitudes of as much as 300 arcsec.