FIGURE 3.1 The minimum detected or detectable signal in flux density versus the year of measurement. The sensitivity is proportional to the temperature of the receiver system and inversely proportional to the collecting area and the square root of both bandwidth and integration time. For measurements after year 1990, an integration time of 12 hours is assumed. The rapid improvement over time is due to system improvements, including the decrease in system temperature (solid-state technology), the increase in collecting area (cost and construction efficiency), and the increase in bandwidth and integration time (electronic and digital technology). The improvement from 1933 to 1983 is about 10 orders of magnitude, a halving time of less than 2 years: a performance improvement similar to that described by Moore’s law. Acronyms in the figure are defined in Appendix F. Figure adapted and updated from J.M. Moran, “Peter Mezger and the Development of Radio Astronomy in the U.S. and Germany, and the Discovery of Radio Recombination Lines,” pp. 475-488 in The Nuclei of Normal Galaxies, Lessons from the Galactic Center, Proceedings of the NATO Advanced Research Workshop, NATO Advanced Science Institutes Series C, Vol. 445, A. Harris and R. Genzel, eds., Kluwer, Dordrecht (1994).