Development of a cold chopper for TAO/MIMIZUKU

摘要

An internal cold chopper is one of the key elements for the ground-based mid-infrared astronomical instruments of the next generation large telescopes such as TMT, GMT, and ELT. For ground-based mid-infrared observations, time variations of radiation from atmosphere, telescope, and instrument are often many times greater than the radiation from targets. Relatively fast (～5 Hz) beam switching technique (chopping) using the oscillating secondary mirror is often used for 8-m class telescopes to remove these background variations, but this is impractical for the 30-m class telescopes due to their much larger secondary mirror. Instead of it, internal chopping mirror is desired for the next generation ground-based mid-infrared astronomy. However, since the mid-IR instruments are usually cooled as low as ～30 K, such internal chopping mirror should also be cooled to that low temperature and low power dissipation of <1 W is required to maintain the low temperature environment, thus it is not so easy to have such cold chopper. As a testbed for that, we are developing an internal cold chopper for the mid-infrared instrument MIMIZUKU for the University of Tokyo Atacama Observatory (TAO) 6.5-m telescope. We use a voice-coil-like structure as actuators and capacitive gap sensors as position sensors. We construct a feedback control system for the rectangular command signal. We made a prototype cold chopper for TAO/MIMIZUKU and are currently evaluating this system. We present the current design of the cold chopper for MIMIZUKU and results of performance evaluation in the laboratory.