LIGO (the famous gravity wave detector) is made of two 4-kilometer arms. According to its website: https://www.ligo.caltech.edu/page/facts https://www.ligo.ca

LIGO (the famous gravity wave detector) is made of two 4-kilometer arms. According to its website: https://www.ligo.caltech.edu/page/facts https://www.ligo.caltech.edu/page/facts > At its most sensitive state, LIGO will be able to detect a change in distance between its mirrors 1/10,000th the width of a proton! This is equivalent to noticing a change in distance to the nearest star (some 4.2 light years away) of the width of a human hair. So I think two telescopes at 450km distance synchronized to "merely" (haha) a visible light's wavelength should be doable, if we throw a fuckton of money on that.