The design and construction of a servo controlled chopper driver for the infrared secondary mirror of the 60 cm reflecting telescope of Beijing Observatory is described. The system allows square wave modulation for a chopping amplitude of 60 arcsec at 11.7 Hz, the 10 - 90% rise time will be 4 - 5 ms with 5 - 10% overshoot without causing vibration or acoustic noise. The system has been used during the infrared observation with a InSb photometer.
Acta Astronomica Sinica
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