Astronomers are constantly battling the scourge of light pollution, retreating to darker and darker observing sites for more sensitive telescopes. What if you are observing with a different kind of light? Are you in the clear? No, as ground-based radio observatories have their own battles to fight against another kind of light pollution: man-made radio frequency interference (RFI).
This week, I arrived at the National Radio Astronomy Observatory's (NRAO) Green Bank Observatory in West Virginia for a two-week stay. This is home to the largest fully-steerable radio telescope in the world, the Green Bank Telescope, or GBT. Astronomers have used this incredibly sensitive instrument to find pulsars, detect interstellar molecules, probe planetary environments, and measure the masses of supermassive black holes. However, our electronic devices give off radio signals, often unintentionally, that could swamp out any astronomical signal with ease. And so, when we visit, we live under strict guidelines.
There aren't many good ways to "see" RFI. In this crude plot, we have the power measured by a radio telescope as a function of frequency, and it should be a smooth curve. All of those spikes are man-made, and that wide feature on the right is digital television.
First, and most strikingly felt by me: Cell phone gets turned off. Yes, for two weeks, I am smart phone-less! There is no use in having it on when there is no cell service and no wireless Internet. Both of those transmit at radio frequencies where astronomers like to do their research. We also have to keep plenty of ethernet cords on hand to plug into the nearest jack. This morning, we ran out in the visitor office and had to find some more.
Microwave ovens can leak radio waves as well, so you won't find them in the observatory dorm's kitchen. The cafeteria has one, but it is shielded in a thick metal box, as is the microwave in the control room that observers can use when snacking well into the night.
The second person to ever do radio astronomy, Grote Reber, discovered that spark plugs from passing cars caused all kinds of interference in his data. Although you can drive your car to the observatory site, you have to switch out for a diesel vehicle if you want to get anywhere near the telescopes. And here is the really fun part... some of these cars have been around almost since the observatory was first built 50 years ago.
I don't care what you say... I love the Scout!
If you want to visit the GBT and the rest of the wonderful telescopes here, please do! There are always tours... just don't expect to use your digital camera anywhere near the telescopes. Once on the bus tour, it's film or nothing. The charged-coupled device, or CCD, inside your digital camera gives off lots of radio waves when you click, and the telescope can see that from far away. (You might sometimes get away with it on a maintenance day when no telescopes are observing if you work here.)
Although the most stringent restrictions are applied to on-site visitors and the surrounding town, the National Radio Quiet Zone allows the NRAO to have some say over transmitters placed anywhere in a 13,000 square mile area approximately centered on Green Bank. It was established in 1958, long before our modern wireless age, in an area with not many people.
Many cell phone companies that have wanted to service the area have had to settle for low-power towers or design them to transmit only in certain directions away from the observatory. Although "back in the day" you couldn't get a radio station signal out here, we've noticed that a few do come in weakly over our car radios.
Now, you may be wondering, don't the radio telescopes give off radiation that they themselves detect as interference? Why yes, self- interference is a serious issue, and one I'll describe in the fascinating conclusion to "The Quiet Zone..."
Leading image credit: NRAO/AUI
Tags: Astronomy, Astrophysics, Space Technology, Telescopes
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