And so the mirror can only tilt but cannot go down.
着镜子只能倾斜，而不能下降。
2
The Michelson interferometer system which has two-beam interference is applied in Fourier Transform interferometer, the moving mirror could tilt in three directions in motion.
If the reference mirror has tilt and piston errors caused by phase-shifting, the phase shifts of the pixels along the same interferogram are not a constant, but they are still kept on the same plane.