By Flavio Falcinelli
The history of radio astronomy began by chance in December 1931: Karl Jansky, an engineer busy in studying the origin of atmospheric disturbances that made long-distance radio communication difficult, using an antenna (known as "carousel" because of its characteristic shape) and a specially designed radio receiver, discovered the radiation at 20.5 MHz from the center of our galaxy as a "byproduct" of his early work.
Block scheme of Jansky’s receiver (“Directional Studies of Atmospherics at High Frequencies” - K. Jansky, 1932)
The first historical radio astronomic record (“Electrical Phenomena that apparently are of Interstellar Origin” - K. Jansky, 1932).
The images (taken from the original articles of 1932) show the structure of Jansky’s instrument and his recordings, where you can see the periodic repetition of the emission peaks from Sagittarius' celestial region, in the center of our galaxy. This was the first (natural…) extraterrestrial radio signal discovered.
The prototype of the modern telescope, seen as a tool built "ad hoc" to observe the sky, was conceived a few years later by Grote Reber. He compiled a map of our galaxy at 160 MHz: by graphing lines with identical received signal strength in function of the antenna orientation, he thus achieved the first "radio-image" of the sky.
The first radio-map of the galaxy compiled by Reber.