Prototype tracking system for observing the Sun at decimeter and meter wavelengths

Abstract

The outer atmosphere of the Sun is understood to be a very hot plasma medium. The frequency of the electromagnetic emission from a hot plasma medium depends on its electron density distribution. Since the electron density is found to decrease from inner to outer layers in the solar corona, high frequency waves originate from the inner corona and low frequency waves originate from the outer corona. It has also been observed that different types of radio emission originate at different heights in the solar atmosphere. Therefore, observing the solar atmosphere over a broadband is expected to give useful insights on the origin, evolution of different types of bursts and their association with other forms of solar activity, etc. Interferometric arrays are common in radio astronomy as the resolution achieved by a single telescope is not comparable with that of those achieved using optical telescopes due to longer wavelengths. A basic radio interferometer set-up consists of two radio telescopes separated by a distance ‘d’ along with a receiver (analog and digital) system. The resolving power of this system is proportional to the distance of separation. As the source moves on the celestial sphere, the incident electromagnetic wave will cause an interference pattern that can be recorded in a suitable receiver system. The existing antenna arrays at Gauribidanur Radio Observatory (Lat: 13◦36’N Long:77◦ 26’E) have antennas mounted vertically upward (i.e. pointing towards local zenith). In this orientation, the direction of maximum reception is towards zenith. To observe the sources at different declinations, the response of the antenna (or beam) is tilted electronically by means of DSBs (Delay Shifter Boards). But, since the hour angle of the source changes continuously over the course of a day, the use of DSBs would be very limited. If any events were to occur before or after the observation time, the instrument cannot observe that. To overcome the above problem, a prototype 2 element interferometer system with tracking facility was proposed to track the Sun and other celestial sources. The planned frequency range of operation was 200-600 MHz. This band was decided for observations because it would cover the coronal region wherein most of the sources responsible for transient radio emission are located. This M.Tech thesis covers in detail the design, development and characterisation of vari ous modules that compromise the system: the analog front-end receiver and digital back-end receiver systems. The analog front-end system consists of the Log-Periodic Dipole Antenna, RF filters, mixers, amplifiers, etc. and the digital back-end system has a Field Programmable Gate Array (FPGA) based receiver. Also it includes the tests performed to select a suitable mo tor for designing the tracking system. Preliminary observations carried out with the prototype observing set-up (without the tracking system) are discussed to show its observing frequency.