Introduction

Radio astronomy is the study of the sky at radio wavelengths. While optical astronomy has been a field of study from time immemorial, the ``new'' astronomies viz. radioastronomy, X-ray, IR and UV astronomy are only about 50 years old. At many of these wavelengths it is essential to put the telescopes outside the confines of the Earth's atmosphere and so most of these ``new'' astronomies have become possible only with the advent of space technology. However, since the atmosphere is transparent in the radio band (which covers a frequency range of 10 MHz to 300 GHz or a wavelength range of approximately 1mm to 30m) radio astronomy can be done by ground based telescopes (see also Chapter 3).

The field of radioastronomy was started in 1923 when Karl Jansky, (working at the Bell Labs on trying to reduce the noise in radio receivers), discovered that his antenna was receiving radiation from outside the Earth's atmosphere. He noticed that this radiation appeared at the same sidereal (as opposed to solar ) time on different days and that its source must hence lie far outside the solar system. Further observations enabled him to identify this radio source as the centre of the Galaxy. To honour this discovery, the unit of flux density in radioastronomy is named after Jansky where

$$1~{\rm Jansky} =10^{-26} W m^{-2} Hz^{-1}$$ (2.1.1)

Radio astronomy matured during the second world war when many scientists worked on projects related to radar technology. One of the major discoveries of that period (made while trying to identify the locations of jamming radar signals), was that the sun is a strong emitter of radio waves and its emission is time variable. After the war, the scientists involved in these projects returned to academic pursuits and used surplus equipment from the war to rapidly develop this new field of radioastronomy. In the early phases, radioastronomy was dominated by radio and electronic engineers and the astronomy community, (dominated by optical astronomers), needed considerable persuasion to be convinced that these new radio astronomical discoveries were of relevance to astronomy in general. While the situation has changed considerably since then much of the jargon of radio astronomy (which is largely borrowed from electrical engineering) remains unfamiliar to a person with a pure physics background. The coherent detection techniques pioneered by radio astronomers also remains by and large not well understood by astronomers working at other wavelength bands. This set of lecture notes aims to familiarize students of physics (or students of astronomy at other wavelengths) with the techniques of radio astronomy.