Noise Temperature in Radio Astronomy

By Mike F.

A little background: Radio astronomers use temperature to describe the strength of detected radiation. Any body with a temperature above -273 deg C (approximately absolute 0) emits electromagnetic radiation (EM).This thermal radiation isn't just in the infrared but is exhibited across the entire electromagnetic spectrum. (Note: it will have a greater intensity (peak) at a specific area of the EM spectrum depending on its temperature). For example, bodies at 2000 K (Kelvin), the radiation is primarily in the infrared region and at 10000 K, the radiation is primarily in the visible light region. There is also a direct correlation between temperature and the amount of energy emitted, which is described by Planck's law.

When the temperature of a body is lowered, two things happen. First, the peak shifts in the direction towards the longer wavelengths and second, it emits less radiation at all wavelengths.

This turns out to be extremely useful. When a radio astronomer looks at a particular point of the sky and says that it has a noise temperature of 1500 K, he/she isn't declaring how hot the body (nebulae, etc) really is, but is providing a measurement of the strength of the radiation from the source at the observed frequency. For example, radiation from an extra solar body may be heated from a nearby source such as a star.If this body is radiating at a temperature of 500 K, it exhibits the same emissions across all frequencies that a local test source does. The calculated noise figure will be the same across all frequencies. (Note: this does not take into account other sources of radiation such as synchrotron radiation).


So, here's the rub. Not only does the source that is of interest to the radio astronomer emit thermal radiation but also both the local environment (ground, atmosphere, etc) and the equipment (antenna, amplifiers, cables, receiver, etc) being used to make the measurements. To accurately observe and measure the distant sources, the radio astronomer must subtract all of the local environment and detection equipment noise additions.

In 1963, Arno Penzias and Robert Wilson were working with a horn antenna trying to make it work with as high efficiency as possible for the Telstar project. This antenna was also going to be used for radio astronomy at a later date. They pointed it to a quiet part of the sky and took measurements.When they subtracted all of the known sources of noise, they found approximately 3 K left over. They worked very diligently to eliminate/describe this noise source and were unable to. This mysterious source of noise seemed to be there no matter where they pointed the antenna. What they had discovered was the microwave background produced from the Big Bang. This 3 (closer to 2.7) K microwave background originated approximately 300,000 years after the Big Bang itself had occurred.It has been determined that when these signals originated, the universe had already cooled down to around 3000 K.


I grabbed this off the web(see ) :

"Given an effective radiated power of the transmitter (in watts), the effective area of the receiving antenna (in square meters), the excess receiver noise temperature of the receiver used (in K), the averaging time of the receiver (in seconds), and the accepted band-width of the signal (in Hz), the range at which we can detect a signal transmitted by an intelligent civilization, is 
R=8x10-6(PeA/T)1/2(t/B)1/4 light years.

where the constant is calculated from 1/[9.4608x10 15(4pik) 1/2]. Here the constant is the number of meters per one light year, and k is the boltzmann constant. "


What makes you think you can discover anything? Who are you?

Nobody. Nobody at all. But the secrets of the universe don't mind. They reveal themselves to nobodies who care. Isaac Newton was a nobody. Michael Faraday was a bookbinder's apprentice....The big laboratories spend millions of dollars, and they work slowly and surely, and they get results. But not the big steps. Those come from the human mind. Not from the laboratory. Call the inspiration, call them intuition, maybe blind luck, Maybe it's God, saying, "Now's the time".

The planet earth is a speck of dust, remote and alone in the void. There are powers in the universe inscrutable and profound. Fear cannot save u. Rage cannot help us. We must see the stranger in a new light-the light of understanding. And to achieve this, we must begin to understand ourselves, and each other.