Introduction

Amongst the various kinds of sources observed in Radio Astronomy, pulsars are perhaps the most unique kind, from many points of view. A pulsar is a neutron star - the ultra-dense core that remains after a massive star undergoes a supernova explosion - spinning at very rapid rates ranging from once in a few seconds to as much as $\sim$ 1000 times per second. A pulsar has a magnetosphere with a very high value of the magnetic field ( $\sim \,10^{6} - 10^{9} \,$ Gauss). The emission mechanism (which is not understood yet) produces radio frequency radiation that comes out in two beams, one from each pole of the magnetosphere. These rotating beams of radiation are seen by us whenever they intersect our line of sight to the pulsar, much like a lighthouse on the sea-shore. Each rotation of the pulsar thus produces a narrow pulse of radiation that can be picked up by a radio telescope. Several properties of pulsars - such as their ultra-compact size, the occurrence of narrow duty cylce pulses with highly stable periods, intensity fluctuations on very short time scales and high degree of polarisation of the radiation - make for a set of observation and data analysis techniques that are very different from those used in radio interferometry. Here we take a look at these special techniques in some detail.