Discovery of Cosmic Radio Waves
Karl Guthe Jansky was born in Norman Oklahoma October 22, 1905 (d.Feb.14, 1950),
graduated with a degree in physics from the University of Wisconsin, and joined
the staff of the Bell Telephone Laboratories in Holmdel, NJ, in 1928. Bell Labs
wanted to investigate using "short waves" (wavelengths of about 10-20
meters) for transatlantic radio telephone service. Jansky was assigned the job
of investigating the sources of static that might interfere with radio voice
transmissions. He built an antenna, designed to receive radio waves at a frequency
of 20.5 MHz (wavelength about 14.5 meters). It was mounted on a turntable that
allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round".
By rotating the antenna, one could find what the direction was to any radio signal.
After recording signals from all directions for several months, Jansky identified three
types of static:
1. nearby thunderstorms, 2. distant thunderstorms, and 3. a faint steady hiss of
unknown origin. Jansky spent over a year investigating the third type of static.
It rose and fell once a day, leading Jansky to think at first that he was seeing
radiation from the Sun.
But after a few months of following the signal, the brightest point moved away from
the position of the Sun. The signal repeated not every 24 hours, but every 23 hours
and 56 minutes. This is characteristic of the fixed stars, and other objects far
from our solar system. He eventually figured out that the radiation was coming
from the Milky Way and was strongest in the direction of the center of our Milky
Way galaxy, in the constellation of Sagittarius. The discovery was widely
publicized, appearing in the New York Times of May 5, 1933.
The Very Large Array is one of the world's premier astronomical radio
observatories, consists of 27 radio antennas in a Y-shaped configuration on the
Plains of San Agustin fifty miles west of Socorro, New Mexico. Each antenna is
25 meters (82 feet) in diameter. The data from the antennas is combined electronically
to give the resolution of an antenna 36km (22 miles) across, with the sensitivity
of a dish 130 meters (422 feet) in diameter. There are actually 28 antennas and
the 28th is always the one that is offline and being refurbished.
History of the VLA:
1972 August: approved by Congress
1973 April: construction started
1975 September 22: first antenna put in place
1976 February 18: first fringes
1980 Formal dedication of the VLA
How Does it Work?
The VLA is an interferometer; this means that it operates by multiplying the
data from each pair of telescopes together to form interference patterns. The
structure of those interference patterns, and how they change with time as the
earth rotates, reflect the structure of radio sources on the sky: we can take
these patterns and use a mathematical technique called the Fourier transform
to make maps.
Who Uses It?
The VLA is used primarily by astronomers from around the world. It's also occasionally
used for atmospheric/weather studies, satellite tracking, and other miscellaneous science.
Each antenna: 25 m (82 ft) in diameter, 230 tons.
The array: there are four configurations: A array, with a maximum antenna
separation of 36 km; B array -- 10 km; C array -- 3.6 km; and D array -- 1 km.
The telescopes are switched between these configurations every four months or so.
Resolution: 0.04 arcseconds
The resolution of the VLA is set by the size of the array -- up to 36 km (22 miles)
across. At our highest frequency (43 GHz) this gives a resolution of 0.04 arcseconds:
sufficient to see a golf ball held by a friend 150 km (100 miles) away. Of course,
very few golf balls contain high-power radio transmitters...
The astronomers use a time called Sidereal time.
This is simply the right ascension of stars on your local
meridian at any moment. Sidereal time runs about 4 minutes a day faster than all
the time systems described above. An old trick is to adjust a wind-up clock to
run 4 minutes a day fast, set it to local sidereal time, and use it to tell what
constellations are on the meridian and what star charts to use. For instance,
if the clock reads 5:30 a.m., right ascension 5h 30m is on your meridian, and
there you'll find Orion.