In the years following the discovery of Sco X-1 (1962), a central problem hindered progress in the nascent field of X-ray astronomy: the positions obtained by rocket detectors had too large an angular uncertainty to confidently identify which visible object, if any, corresponded to each X-ray source. Japanese physicist Minoru Oda developed a modulation collimator that for the first time allowed narrowing down Sco X-1's position to a region of the sky small enough to attempt an optical identification. With this improved position, an international team led by astronomer Allan Sandage (Mount Wilson and Palomar Observatories), which included Riccardo Giacconi, examined the region and found a faint, erratically bright blue star with unusual emission lines, resembling those of old novae. Later named V818 Scorpii, this star turned out to be the optical counterpart of Sco X-1: the first time a cosmic X-ray source was linked to an object observable through conventional optical telescopes. The finding, published in 1966, opened the door to multi-wavelength astronomy of X-ray sources and laid the groundwork for, years later (with work by Gottlieb, Cowley, and Crampton in 1975), determining that Sco X-1/V818 Sco is a compact low-mass binary system, with a neutron star accreting matter from a companion, with an orbital period of 18.9 hours. The 1966 identification did not yet imply this detailed binary nature, but it was the indispensable step that made it possible.