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 <100 mW.Another difficulty is the variable atmospheric absorption. In order to measure the brightness and spectrum of an object, it is necessary to observe a reference object with known brightness and spectrum, and compare them to correct for the atmospheric absorption. The best way to accurately correct for the time-varying atmospheric absorption is to observe the two sources simultaneously. Such observations are difficult due to the small number of objects observable from the ground, but they are very important for monitoring that requires accurate atmospheric correction. To overcome this problem, we have developed a system called Field Stacker (FS) and installed it on the MIMIZUKU. This system makes it possible to pick up arbitrary two objects in the telescope field-of-view (FoV) and observe them simultaneously. We confirmed in test observations at Subaru that this kind of observation actually improves the accuracy and reliability of photometric and spectroscopic data.In this talk, I will talk about these efforts to overcome the difficulties of ground-based MIR observations and future prospects of observations with TAO/MIMIZUKU.