NASA's Goldstone Deep Space Antenna

Radio Astronomy: Telescopes, Detection, Interferometry, Resolution, Drawbacks, Etc.

Radio astronomy is the sub-field of astronomy that studies celestial objects at radio frequencies.

The First Detection

Karl Jansky.
Karl Jansky. Credits – Saber1983. Source. CC BY-SA 3.0.

The first detection of radio waves from an astronomical object was in 1932 when Karl Jansky at Bell Telephone Laboratories observed radiation coming from the Milky Way.

Radio astronomy is conducted using large radio antennas referred to as radio telescopes, that are either used singularly or with multiple linked telescopes utilizing the techniques of radio interferometry and aperture synthesis.

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The Usefulness of Interferometry

The Atacama Large Millimeter Array (ALMA) in Chile.
The Atacama Large Millimeter Array (ALMA) in Chile. Credits – ESO. Source. CC BY 4.0.

The use of interferometry allows radio astronomy to achieve high angular resolution, as the resolving power of an interferometer is set by the distance between its components, rather than the size of its components.

At any given time, the telescope is receiving radiation from a piece of sky determined by the diffraction pattern of the antenna. If we want to build up a radio image of part of the sky, we must point the telescope at each position and take a separate observation. Recently, improvements in receiver technology have allowed limited multi-receiver systems.

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How to Increase Resolution

The Goldstone Deep Space Antenna in the Mojave Desert, California, USA.
The Goldstone Deep Space Antenna in the Mojave Desert, California, USA.

Radio telescopes may need to be extremely large in order to receive signals with a low signal-to-noise ratio.

The difficulty in achieving high resolutions with single radio telescopes led to radio interferometry.

Modern Radio interferometers consist of widely separated radio telescopes observing the same object that are connected together using coaxial cable, waveguide, optical fiber, or other types of the transmission line. This not only increases the total signal collected, but it can also be used in the process called aperture synthesis to vastly increase resolution.

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The Drawbacks

Observations from the Earth’s surface are limited to wavelengths that can pass through the atmosphere. At low frequencies or long wavelengths, transmission is limited by the ionosphere, which reflects waves with frequencies less than its characteristic plasma frequency.

Water vapor interferes with radio astronomy at higher frequencies, which has led to the building of radio observatories that conduct observations at millimeter wavelengths at very high and dry sites, in order to minimize the water vapor content in the line of sight.

Also see Betelgeuse is Surprisingly Smaller, Closer to Us.

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