Schematic view of the six-mirror bowtie fsEC setup. The inset at the upper right shows the IR-viewer photograph of the fsEC under air condition. The acronyms in the figure are given by: HV, high voltage; FG, function generator; PZT, piezo transducer; CCD, charge coupled device; PD, photodetector; IC, input coupler; PID, the proportional-integral-derivative circuit; MA (MF), mirror A (mirror F).