Autofocusing system based on optical astigmatism analysis of single-molecule images.

Single-molecule fluorescence imaging has greatly contributed to our understanding of many bio-molecular systems. While reactions occurring in the range of several minutes can be readily studied using conventional single-molecule fluorescence microscopes, data acquisition for longer time scales is hindered by the focal drift of high numerical aperture objectives, which should be corrected in real time. Here, we developed a robust autofocusing system based on optical astigmatism analysis of single-molecule images. Compared to the previously developed methods, our approach has a merit of simplicity in that neither fiducial makers nor an additional laser-detector system is required. As a demonstration, we observed B-Z transition dynamics occurring for several hours.