The wavelengths of radio waves are very large, and as a result large dishes are required to "capture" them. Because radio is of a lower energy than visible light, radio waves can reflect off of non-reflective surfaces. As a matter of fact, a disk does not have to be completely solid since the waves are so large.

In 1931, Karl Jansky was tasked by Bell Telephone Labs to determine the cause of static over long distance telephone lines. He built a device in an attempt to pick up stray radio waves.

Instead, by 1933 he realized the emission was coming from space along the path of the Milky Way galaxy. By 1935, the first radio disk was constructed by Grote Reber, and radio astronomy was born.

While radio astronomy is used to study a wide variety of topics, the most common use is the mapping of hydrogen emission. Such emissions allowed for the determination of the spiral structure of our galaxy - using Doppler shift.

Hydrogen is the most abundant element in the Universe. It exists scattered throughout the Universe as well as members of larger clouds of dust and gas within galaxies. The hydrogen atom consists of 1 proton and 1 electron, and exist in one of two states: aligned or opposed:

The rotation (spin) of the electron can spontaneously shift to the opposite direction releasing a photon. This energy is detected at 21cm, or 1.42GHz (1420MHz). This shifting occurs about every 400 years for a single hydrogen atom, but is detected often due to the abundance of hydrogen.

The radio dish is the tool used by astronomers to study this region of the radio spectrum. Generally the larger the dish the better, but more novel approaches have been used to create a virtual dish by creating a large array of radio dishes like those at the VLA in Socorro New Mexico. Even radio dishes from other countries are tied together creating the Very Long Baseline Array - or VLBI. The image on the right is a dish called a Cassegrain, just like the telescope. The radio wave bounces off the big dish, to the secondary dish at the top, then through the center to the radio gear at the base of the tower.

So what does a radio image look like? Just a bunch of numbers really. Astronomers apply their collected data and plot them based on intensity and assigning them a color. These images of spiral galaxy NGC253 are a good example:

This is an optical image.	This is the radio "image."

The red on the radio image indicates higher levels of 21cm emission while the violet indicates the lowest levels. Strange really since the opposite holds true for stars - red stars are cooler than blue stars, so be careful with the details.

Back to Top