Back To CourseBasics of Astronomy
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You know what's really amazing? Despite all the technological innovations in TV, computers, the Internet and what not, the radio is still alive and well today. Some scholars believe it's because when we listen to the radio, we believe it's speaking directly to us, making us feel warm, fuzzy, and important deep down.
I don't know the real reason for why the radio has stuck around, but I do know that the radio waves used in radios aren't just for sound. They're for looks too. Telescopes, called radio telescopes, rely on radio waves as much as any radio you may have.
This lesson will go over what radio telescopes are, the basics of how they work, and some other key concepts related to their use.
Radio telescopes are instruments used to detect, collect, and analyze radio waves coming from cosmic sources. If you look at the electromagnetic spectrum, you can see that radio waves include a very large range of frequencies. As a result, radio telescopes vary a lot since different techniques must be used for different parts of this spectrum. Here's just one example of what I mean.
Radio telescopes are reflectors. This means they use metallic surfaces that act as mirrors. These mirrors reflect radio waves to a focus. In order to accomplish this task well, the surface of a radio telescope may need to be very smooth. How smooth? The imperfections on the telescope have to be smaller than 1/10 the wavelength the telescope is tuning into.
What this means is, radio telescopes that are designed for wavelengths that are longer than a meter can have tennis sized holes and bumps in them and still reflect really well. In fact, you can just use some chicken wire mesh for the longer-wavelength radio waves. Obviously, if the radio telescope is operating at millimeter-long wavelengths, then chicken wire won't work and the irregularities on the telescope's surface have to be very small.
I'm sure you can appreciate why this is the case with a much simpler example closer to home. If you want to catch a big and long fish in a net, you can have big holes, and you'll still catch it. But that won't work for shorter and smaller fish, which will slip right through. So, in that case your net will need to have holes that are a lot smaller to catch the smaller fish.
Radio astronomers, the poor fellows, have two big disadvantages when compared to optical astronomers. Another lesson would've taught you that the resolving power of a telescope depends on two key factors. One was the size of the primary mirror or lens and the other was the wavelength of radiation in question. The longer the wavelength, the more diffraction, and the worse the image.
Thus, when it comes to radio waves, the diffraction fringes will be large and the images produced by radio telescopes aren't as detailed as those produced by optical telescopes of a similar size.
Another problem radio astronomers face is the fact that longer wavelengths of radiation have lower energies. This means that radio signals arriving from the universe are very weak and radio astronomers need to either build humongous single dishes or combine lots of smaller dishes together before amplifying the signal further to be properly measured.
The largest single radio telescope has a diameter of 1,000 feet and a circumference of about 1,000 meters! And if you wanted to resolve the details of a galaxy as well as a much smaller optical telescope, you'd have to build a radio dish that's the size of Rhode Island.
So, to get a high angular resolution, radio astronomers have turned to interferometry. I sort of glossed over what this is just a second ago.
Interferometry is the use of more than one telescope, connected together and operating as one instrument, in order to achieve a higher angular resolution. Angular resolution is a telescope's ability to see two glowing objects as distinct and separate sources of light. In interferometry, widely spaced radio dishes produce a resolution that's like the resolution of one telescope that's as large as the distance between the two dishes in question.
As a real-world example of this, there are two telescopes, called Keck I and Keck II on top of Mauna Kea. They are 85 meters apart. When they are used as an interferometer (a radio telescope made of two or more separate antennas), the angular resolution is equivalent to one 85-meter telescope. If your eyes were this good, this would allow you to read the last row on an eye-chart 22 miles away!
Radio telescopes are instruments used to detect, collect, and analyze radio waves coming from cosmic sources. They are a kind of reflecting telescope, and therefore, use mirrors to reflect radio waves to a focus. Although you can technically build a radio telescope out of cheap chicken wire, radio astronomy does have some serious disadvantages compared to the expensive optics of optical astronomy.
Namely, this lesson went over the fact that longer wavelengths of radiation suffer from more diffraction, and that radio waves are of low energy. Thus, radio astronomers need to either build large dishes or combine several telescopes in a process called interferometry to try and compensate for these problems.
Interferometry is the use of more than one telescope, connected together and operating as one instrument, in order to achieve a higher angular resolution. Angular resolution is a telescope's ability to see two glowing objects as distinct and separate sources of light.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons