Radio astronomy began, randomly, with the discovery of cosmic radio emission occurred in December 1931 by Karl Jansky, a young engineer at Bell Telephone Laboratories while studying at Holmdel (New Jersey), the origin of atmospheric disturbances that interfered with the first commercial radio communications over long distances.
Immediately after graduating from the University of Wisconsin in 1927, Jansky began his first professional experience at Bell Telephone Laboratories, first in Cliffwood, then in Holmdel New Jersey: he had obtained the important office, supervised by H. T. Friis, to study the nature of the disturbances of atmospheric origin that created considerable problems of reliability to transatlantic radio telephone. At that time they had been sufficiently studied the effects of radio noise of natural origin on radio communications in frequencies below 10 MHz, while little is known about the practical implications of the new technology of "radio waves court".
The first rudimentary Radio Telescope (1931): in the initial plans, it was a tool developed for the analysis of radio interference that hindered the first international radiocommunication. It's the famous "carousel Jansky" operating at a frequency of 20.5 MHz
The structure, able to rotate 360°, was organized as an array of square loop antennas connected to a receiver specially constructed.
Was designed and built a receiver system composed of an antenna system to arrays of square loops (see the figure above), able to produce reasonable characteristics in directivity, physically adjustable in azimuth so as to determine with certainty the direction of arrival of the interference. The receiver, operating at a frequency of 20.5 MHz with a bandwidth equal to 1 KHz, must ensure, besides a good sensitivity, also a sufficient stability, minimizing the fluctuations and long-term drifts of the gain or level of zero reference of the final stage of measurement (was adopted an integration time of the output signal equal to 13 seconds). These are the characteristics of the receiving system that allowed Jansky to highlight the cosmic radiation from the center of the galaxy compared to the background noise of the equipment.
The origin of the broadcasting seemed to coincide with the celestial region of Sagittarius (galactic center): following observations carried out in the following years confirmed that the radiation was concentrated along the band of the Milky Way galaxy, with a maximum intensity towards the center.
Although the relative simplicity of the equipment Jansky has detected the emissions of the galactic center thanks to the optimization of some parameters of the receiving system (sensitivity, stability and long time constant of integration of the output signal), thanks to the high intensity (at a frequency operative choice) of galactic radiation and thanks to the considerable angular size of the emitting region (the brightness temperature associated with the galactic center, at a frequency of 20 MHz, is equal to approximately 200000°K).
The equivalent noise temperature of the antenna of the instrument of Jansky was of the same order of magnitude of the brightness temperature of the source, while the noise figure of the receiver, compared with that of existing tools, was not particularly good (approximately 17 dB): however, the average level of the received signal is maintained several times greater than the level of background noise.
Reber builds the first real radio telescope
The first real instrument dedicated to astronomy, considered the prototype of the modern telescope was designed and built by Grote Reber (1937). It was a parabolic reflector with a diameter of 9.5 meters assembled on a strong wooden frame and connected to a receiver of good sensitivity operating at a frequency of 160 MHz
The width of the antenna bound at half power was of the order of 12°. The system (instrument transit the meridian) was able to scan in different areas of the celestial sphere by varying the angle of the antenna (declination) relative to the horizon, and exploiting the rotation of the earth.
After he has collected and processed a sufficient number of data in 1944 Reber was able to fill in the first radio map the Galaxy graphing isotherms lines of the temperature distribution of sky brightness at 160 MHz.
He watched as the maximum intensity of the main emission occurs near the galactic center (Sagittarius constellation), confirming the earlier work of Jansky, while many secondary maxima were located in Cygnus, Cassiopeia, Canis Maior and Puppis, with a maximum of lower intensity in Perseus.
Reber was a highly skilled technician who managed to improve his equipment to obtain the best performance that the radio technology was able to provide at the time. He pointed out that the whole antenna-receiver (the radio telescope) behaves as a bolometer, a device used for radiometric temperature measurements: the radiation resistance of the antenna is the "sensor" that reads the temperature of the equivalent regions of the sky intercepted by its reception cone.
Inspired by these results Reber tried the reception at frequencies significantly higher, first to 3.3 GHz and 900 MHz, with no result. This was due to decreased sensitivity of the receiver at high frequencies and to the fact that, in that interval spectral, intensity of cosmic radiation is not particularly high.
Emissions revealed by Jansky and Reber corresponded to a contained type radiation.
During the Second World War Oort and Van de Hulst (University of Leiden) were to know of the work of Jansky and Reber and wondered if it would be possible to observe a few lines in emission (similar to studies in the field of spectroscopy) highlighting the fact that if all the radiation were concentrated in a narrow frequency band, instead of being distributed over a broad spectrum, the signal would be much more intense and much easier to detect. Van De Hulst then studied mechanisms that could generate a row and in 1944 announced that the interstellar neutral hydrogen could be a good candidate.
The discovery of neutral hydrogen in emission (at a frequency of approximately 1420 MHz, known as row 21 cm) did, indeed, March 25, 1951 at Harvard by Ewen and Purcell.
Since 1950 the development of radio astronomy was very fast and full of discoveries, all of fundamental astrophysics and cosmology interest. The discovery of the hydrogen emission line at 21 cm gave immediately start searching for other substances in the interstellar medium, work that, to date, has led to the cataloging of a large number of complex molecules many of which are organic.
The observations of the 21 cm line represent, a good title, the first self-contained and highly successful radio astronomy as they have shown for the first time the spiral structure of the galaxy, the optical observation precluded due to the absorption of interstellar clouds .
Significant was the patient work that has led to an accurate measure of the position of many radio sources, most of which were subsequently identified with optical counterparts.
In 1963 came the discovery of quasars (Quasi Stellar Radio Sources): far away extragalactic objects (at the edge of the observable Universe) with much smaller than normal galaxies but considerably more issuers in the optical domain and in the radio.
In 1965 it came, randomly, the discovery of the background radiation at 2726°K.
Working on the development of radar technology in the '40s, R. Dicke MIT invented the microwave radiometer (the so-called Dicke-Switch), a very stable receiver capable of detecting microwave radiation of low intensity. In the '60s R. Wilson and A. Penzias, two engineers of the Bell Telephone Co. engaged in the study of the causes of noise that disturbed the first broadcasting via satellite, used the Dicke receiver to create a tool that would track satellites for telecommunications Echo 1 and Telstar.
Working at a frequency of 4.08 GHz with a large horn antenna (opening of 6.2 meters) they discovered that the instrument measured unexpected radiation: performing a scan in all directions of the space is recorded a constant background noise (equivalent to an absolute temperature approximately 2.7°K) independent of the direction of antenna pointing and time of observation. The interpretation of this radiation is isotropic, which earned him the Nobel Prize to the discoverers, they were suggested by the news that their Dicke had previously proposed to use a microwave radiometer to measure the cosmic microwave background radiation.
This discovery is of fundamental cosmological importance: isotropy of the background radiation (which uniformly fills the entire space) appears to be the fossil residue caused by the subsequent expansion of the radiation that permeated the universe in its very first moments of life, as predicts the cosmological theory "Big Bang".
Later were performed numerous checks and measures on the background radiation, confirming the characteristic property of isotropy and the fact that its spectrum is very similar to a black body in thermal equilibrium at a temperature of 2.726°K.
The temperature of 2.7°K determines an ultimate limit to the sensitivity for all radio telescopes.
The discovery of the Pulsar
In 1968 there was an extraordinary event for radio astronomy: the discovery of pulsars (Pulsating Radiosource: PSR) by A. Hewish and J. Bell Burnell at the Observatory of Cambridge.
In 1965 Hewish began the construction of a plane rectangular array composed from 2048 dipoles positioned horizontally at some wavelengths of height from the ground and operating at a frequency of 82 MHz. The physical surface of the instrument covered an area equal to approximately 20000 square meters and had the ability to move in declination (a few degrees) its main lobe modifying the phase of the signals collected by the various groups of elements that arrived at the receiver (by varying the lengths of the lines of coaxial cable that constitute the transmission lines of the antenna). The Earth's rotation ensured scanning in right ascension.
The plant was designed to study the compact radio sources by analyzing the quasar phenomenon of scintillation produced by their irregular structure of the interplanetary medium. While the researchers were engaged in this work noted in their recordings, considerable and regular fluctuations of the signal. Optimizing the time constant of the receiver (very short) managed to highlight a series of pulses characterized by regular repetition period and theoretically generated by objects in the final stage of evolution, characterized by a radius of between 10 and 50 km and mass between about 0.5 and 2 solar masses: they were the pulsar (or neutron stars). The width of the pulses of the first pulsar discovery was of the order of a few hundredths of a second.
These pulses, with different shape according to the radio source, have very small width compared to the repetition period (a few percent) and are of complex shape, which reveals details of the duration of 0.2 milliseconds or less. The neutron stars are observable over a wide frequency band even if their emission is more intense toward the longer wavelengths (typically in the VHF band) with a rapid decrease in intensity toward the higher frequencies.
Since the mid-60s until today, stimulated by the enormous development of electronic technology, astronomy research has been characterized by a continuous discovery of spectral lines in the radio domain (in particular in the range of microwave and millimeter wave) due to interstellar molecules many of which are organic.
From the instrumental point of view it is a gradual development towards radio interferometric techniques, along with the implementation of sophisticated algorithms and software necessary for the processing of the data: they have developed "radio images" increasingly defined and reliable of the observed objects.
About 70% of the current knowledge about the universe and its dynamics are due to radio astronomy observations.