THE SIMPLE ADDING INTERFEROMETER

An interferometer is a combination of two or more radio antennas working together to image the same source. The use of an interferometer is primarily motivated by the need for better resolution. In a radio telescope, the resolution (minimum angle between two point sources that can still be detected to be separate) is equal to the wavelength upon the diameter of the telescope. So, even for a 45 meter size dish of GMRT (Giant Meterwave Radio Telescope, India), the resolution is about 1/45 rad = 1.27 deg, while a small 6” optical telescope has a resolution of 0.00025 deg (about 6,000 times better!) Clearly, we need better resolution in radio telescopes in order to coordinate with other observational astronomers and gain good images of celestial sources. Now, if we were to use a single dish to have resolution of 10-3 degrees at meter wavelengths, the size of the antenna would have to be a few tens of kilometers. This problem is overcome by the use of interferometers, which to some extent simulate a large telescope antenna and thus give better resolution. In terms of the lobe pattern of the antenna, this interferometer as a whole has a narrower main lobe, so is closer to the ideal “impulse response” that we would want from a radio telescope.

The simple adding interferometer consists of two antennas separated by a distance D between them:
This is in some sense like a double slit for observing an interference pattern. Indeed, it can be proved that the pattern we get by observing uniform radiation in the sky using such a device is identical to a double slit diffraction pattern. Let us do some analysis of the interferometer for getting an insight into its working.



The given this output, we can get back the signal if we know the impulse response of our system, i.e. the lobe pattern of the antenna. However, here we note that we have still considered only simplified cases of a one-dimensional scan across a source, while in real astronomy, astronomers are interested in forming 2-D maps of the sky. That adds another degree of complicated mathematics to the process of reconstructing the signal from the output, however, the basic underlying theme remains the same. The actual reconstruction is done from the combined output of several antennas, each with a different baseline (separation), and working together as more advanced types of interferometers. This process is very computation-intensive and professionals use highly specialized hardware to do this job.

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