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
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!