Chopping observations with a tip-tilt secondary mirror have conventionally been used in ground-based mid-infrared observations. However, it is not practical for next generation large telescopes to have a large tip-tilt mirror that moves at a frequency larger than a few Hz. We propose an alternative observing method, a slow-scanning observation. Images are continuously captured as movie data, while the field-of-view is slowly moved. The signal from an astronomical object is extracted from the movie data by a low-rank and sparse matrix decomposition. The performance of the slow-scanning observation was tested in an experimental observation with Subaru/COMICS. The quality of a resultant image in the slow-scanning observation was as good as in a conventional chopping observation with COMICS, at least for a bright point-source object. The observational efficiency in the slow-scanning observation was better than that in the chopping observation. The results suggest that the slow-scanning observation can be a competitive method for the Subaru telescope and be of potential interest to other ground-based facilities to avoid chopping.
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