Interferometry Explained

Measuring Space with Time

The distance to stars is often measured in light years. If a particular star is 10 light years away, it means the light from the star takes 10 years to reach us. The speed of light is constant, so we can use the travel time of light to measure distances. Radio astronomers use this trick to triangulate the precise location of celestial objects.

When two antenna dishes look at an object in the sky, they view it from slightly different positions. This means light from the object reaches one antenna a bit before it reaches the other. The time difference is only a tiny fraction of a second, but it means the timing of the two signals are a bit off. When combined, they would overlap or interfere with each other, blurring the signal. But by precisely timing the signal from each antenna, astronomers can correlate them so they don’t interfere. This is known as interferometry.

With interferometry, radio astronomers can combine the signals from many antennas, and even many telescopes. It allows them to create an image that is much brighter and sharper than what is possible from a single antenna dish.

How to get the best image

Many factors can affect the quality of the radio image. Placing antennas far apart makes the image sharper, while placing them close together makes the image brighter. More antennas gather more light, but also make the telescope more expensive to build and operate. Observing for a longer time is usually better, but telescope time must be shared with other astronomers. Even the way antennas are arranged can affect what we see.

Interferometry in Action

In the below application, you are the astronomer.

Pick an image you want to observe below by clicking on it, then click “Next”
Build a preset or custom array of antennas to use for your observations. Select the shape and the click the plot button or the use the + and – buttons to add or remove antennae.
Click the “Observe” button to simulate the observation results
Click the “Back” button to adjust your configuration and try again
Check the “Compare to previous” box to see how your observation changed with your new configuration

Going Deeper

Experiment with different configurations, make several observations and compare them. Hover or click on underlined termsUnderlined TermsYou can click on or hover over an underlined term below to learn more about that term or how to better experiment with interferometry. to learn more about them.

How does an observation with a “small” configuration compare to one with a “large” configuration?
How does the time of the observation effect the final image you observed?
Can you get a clear image with only a few antennas?
Does your configuration work well for some images but not others?

Test your ideas to see what works!

Shape	# of Antennaes	Size	Plot
Circle
YY ConfigurationAntennas are arranged along 3 lines radiating from the center. The VLA configurations are of this type.
SpiralSpiral ConfigurationAntennas are arranged in a pattern spiraling from the center. Often multiple spirals are used. ALMA often uses this pattern.
3-Arm SpiralSpiral ConfigurationAntennas are arranged in a pattern spiraling from the center. Often multiple spirals are used. ALMA often uses this pattern.
CustomCustom ConfigurationUsing the add and remove buttons as well as clicking and dragging antennae on the plot below, place antennae in whatever shape you'd like. Can you discover a new configuration astronomers might use?

Measuring Space with Time

How to get the best image

Interferometry in Action

Going Deeper

Choose your source imageSource ImageChoose which image your simulated telescope will observe. In radio astronomy the source is a particular radio object in the sky. to observe:

Build an antennae configurationAntennae ConfigurationEach configuration is a pattern showing how the antennas are arranged, including the shape of the pattern and the spacing of the antennas.:

Observation Results:

Previous Observed Sky

Observed SkyObserved SkyThis is the result of your observation. How well did you capture the details of your source image? How might you improve your result?

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
	Founded in 1956, the NRAO provides state-of-the-art radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers regardless of institutional or national affiliation. Observing time on NRAO telescopes is available on a competitive basis to qualified scientists after evaluation of research proposals on the basis of scientific merit, the capability of the instruments to do the work, and the availability of the telescope during the requested time. NRAO also provides both formal and informal programs in education and public outreach for teachers, students, the general public, and the media.
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