Today, Aso-san pointed out that the silicon mirrors may have higher finesse than we expected.
Therefore, I checked the spec sheet of mirrors, and the designed reflectance is more than 99.997%, which corresponds to more than 100,000 finesse, though we ordered as 50,000 finesse.
The manufacturer may assume that the loss (probably ~30ppm) reduces the finesse from ~100,000 to 50,000.
However, it may be almost impossible to see transmitted beam with such high finesse and loss...