The synthesized beam is analogous to the point-spread function in a kind of these devices that measure visibility in u-v space, which is Fourier transformed to generate an image. Test masses suspended from four pendulums reduce seismic noise in one of these devices consisting of two four kilometer-long étalons (“ay-tuh-LON”). Some of these (10[1])devices use aperture synthesis to achieve a resolution on the order of wavelength divided by baseline, which the EHT used to take the first image of a black hole. Two of these devices in Louisiana and Washington detected a 2017 neutron star merger, which was the first gravitational wave signal. The LIGO observatory uses Michelson’s design for these devices. For 10 points, what devices measure the phenomenon that occurs when two waves are superimposed? ■END■