Challenging the difficulties in ground-based MIR observations: The case of TAO/MIMIZUKU

In the JWST era, investigations of faint objects will greatly progress thanks to the unprecedented high sensitivity and spatial resolution of the JWST. The role of ground-based observations in such an era will be survey observations of bright sources and monitoring observations of time-varying phenomena. The mid-infrared instrument TAO/MIMIZUKU, which we are currently developing, aims to play such a role. The TAO (the University of Tokyo Atacama Observatory; PI: Y. Yoshii) is an observatory that the University of Tokyo is constructing at the summit of Co. Chajnantor (5640-m altitude) in the Atacama Desert, Chile. The high altitude and dry climate suppress precipitable water vapor, allowing for stable observations in the MIR and extending the atmospheric window to 38 microns. The TAO will be equipped with a 6.5-meter telescope, and the MIMIZUKU will be the first-generation MIR instrument mounted there. The MIMIZUKU will enable imaging and low-resolution (R=60-600) spectroscopy in 2-38 microns. We aim to use this capability to study the formation, growth, and destruction of dust around young and evolved stars by monitoring the associated temporal changes. There are some difficulties in ground-based MIR observations. One is the strong thermal background radiation from the atmosphere and the telescope. Since the background radiation is much brighter than astronomical sources and is highly variable, it is important to remove it. The simplest way to do this is to switch observing positions before the background level changes, take data at each position, and subtract them. This technique is called "chopping," and the MIMIZUKU will have a chopping system (beam switcher) in its cold optics. We are developing such a system that enables beam switching with a chop throw of >30 arcsec, a frequency of >2 Hz, and a power consumption of