“A compound lens, as its name suggests, is made up of two lenses.”
J1721+8842 looked like one elliptical galaxy bending a quasar’s light into six images. Webb showed that the reddish ring astronomers took for an Einstein ring is a second lensing galaxy. Two years of data on light arriving through two lenses gives a cleaner handle on the Hubble constant. The universe built a better telescope than we did, and we needed Webb just to notice.