Exactly. The imprecision of the mirror surface (and various other optical surfaces in the system) cause their own interference pattern that can be measured and subtracted from the recorded signal.<p>Even so, measuring gravity waves requires ridiculous amounts of precision in the construction of the interferometer. I&#x27;m working at the 10^-6 scale, where optics can still be adjusted by hand. They are working at the 10^-21 scale - the sheer engineering challenge is awe-inspiring.