Radio Telescopes

Does anyone of you accidentally roughly know how a radiotelescope works or rather what kind of data you can get from one? I went through Wikipedia, but what I want to know is what kind of data you receive when an object is detected and how long it would approximately take the object to get “here” once detected at location X - and how that location could be entitled. Any help appreciated.

Roughly how it works is that it’s a radio with a very directional antenna. That’s all. You pick up radio signals with it, which is to say electromagnetic radiation of very low frequencies.

I’m not sure what else you’re asking. If you detect an object with one, it’s because the object was emitting radio signals. The signals travel at the speed of light; how long the object takes to arrive depends on how fast the object moves.

If it’s even heading this way… most space stuff emit radio waves in multiple directions, not just the direction they’re traveling.

The only ones I know about are the Cangaroo project in Woomera (I used to live there and managed to get myself a tour by writing an article for the local paper): … /cangaroo/

My understanding of it was that stars give out radiation (in many different spectrums). The dishes pick up those waves (usually in the gamma band), and the computer interprets them and creates a composite image. Of course it only works at night, because the radiation from the sun floods the telescope. Even the reflected sunlight from the moon creates too much interference, thus the radio telescopes can only be used for about 2 weeks a month (when the moon is dark, and about a week to either side).

PS - these things are monsters, they’re huge! The inside of them is full of computer equipment, so I’m thinking it’s pretty CPU intensive, and of course requires special software.

PPS - those mirrors have to be PERFECT, even more so than normal telescopes. That’s why Cangaroo use lots of small ones, rather than one large one, because they’re easier and cheaper (by millions).

EDIT: These are set up in the middle of the Aussie outback so they don’t get buzzed with lots of Electromagnetic radiation.

As for your questions, as stated above, the info travels at the speed of light. The telescopes are basically picking up what’s out there in space (from that small patch of sky directly in front of it). Apparently it has the potential to be sensitive enough to check if there are planets circling the stars (ie the stars will ‘dim’ as the planet travels in front of them) but whether or not they’ve had any success I don’t know.

EDIT2: - here’s a copy of the article. It was one of the first I wrote, so don’t judge me, but it is in pretty easy to understand terms.

Like the previous posters, I’m not too certain what you are asking for. Just to be sure this is clear, a radio telescope is not a radar: whereas a radar detects objects by emitting radio waves and then detecting the reflections of those waves, a radio telescope doesn’t emit anything. It just sits there waiting for radio signals to be emitted by objects in outer space.

Like other telescopes, a radio telescope does not detect an object at a certain location. The most it can do is detect radio signals coming from a certain direction. If we want to know how far away the object is, we need to use additional techniques. (This, again, is different from radar and sonar, which can check how long it took for their signal to reach the object and come back, and can therefore calculate the distance to the object.) When humans use their eyes, they gauge the distance using stereopsis: the slight differences in how things look to the left eye and the right eye can be used to decide how far the object is away. Unfortunately, for most objects in space, the distance you would need between your two telescopes in order to make a good guess at how far they are away is far larger than the diameter of the earth.

There are a couple of techniques used in astronomy for deciding how far an object is:

  • You can increase the distance between your telescopes by making use of the fact that the Earth revolves around the sun. If you make a measurement in spring and another one in autumn (and the object you’re looking at doesn’t move a lot in six months, compared to how distant it is from us), you have effectively increased the distance between the telescopes to the diameter of the earth’s orbit.
  • If you know how much light (or other electromagnetic radiation) the object emits, you can measure its apparent brightness and decide how far it is away. (The amount of radiation received goes down as the square of the distance.) This only works if you have a correct theory about the kind of object it is and the amount of light such an object emits.
  • Because of the expansion of the Universe, the Doppler red shift can be used to gauge the distance of objects that are very far away (outside our own galaxy).

and there are probably more, but it has been some time since I followed astronomy courses.

The place in the sky at which a radio source is detected is named in the standard astronomical way: Right ascension and declination.

There is absolutely no way to say how long it would take an object to get “here”. Most radio sources will never get here, since they move away from us. A radio source that suddenly appears on our image of the sky could be here almost immediately after: if it travels near the speed of light, it will almost keep up (from our point of view) with the radiation it emits, and will thus arrive shortly after – even shortly after radiation that it emitted many, many years ago. (Although note that an object moving towards us near the speed of light will, by the Doppler effect, have a blue shifted spectrum, making it rather unlikely that we would detect radio waves. Red shift brings radiation closer to the radio end of the spectrum, whereas blue shift brings it closer to the gamma ray end of the spectrum.)

Wow, nice post.

Interestingly enough the Cangaroo telescopes detect radiation in the gamma ranges.

Yeah, gamma rays are not radio waves (very high frequency, not very low frequency). The telescope constructions are different. Radio telescopes don’t need very good mirrors; a sheet of wire mesh will work. They work in the day time, too.

Thanks already. “Speed of light” is great for me to determine when an object will impact, and I’ll have several RTs to make it plausible the location has been detected.
This post was to plug a story hole, in case that was unclear. Thanks again!

How so? Is something coming at the earth at that speed? If so, it’s impossible to see it coming in the first place. (We ignore the fact that it’s not possible for anything that has mass to travel at the speed of light though.)

Hm, no, the object is slower, but the radio waves received by several RTs should enable me to determine the time of impact of the object, right?

Only if it is relatively close compared to the distance between the radio telescopes. I’m sure two telescopes at opposite ends of the earth could easily pinpoint an object as far away from us a Pluto, and probably farther; but perhaps not something that is several dozen light years away.

This is called parallax and can be used to determine distances. Parallax is the “effect” you can see when you put your finger in front of you, close one eye, then the other, then the other again, and see how the finger moves left and right, depending on which eye you’re using to see. This can be used to calculate distances. And indeed that’s what the brain does to make out how far away something is.

The distance between our eyes is only good enough to calculate relatively short distances. The width of the earth (with one telescope at each side of the earth, or by measuring each 12 hours with one telescope, so that it gets to the other side due to earth’s rotation) is good enough to make out distances in our own solar system. For objects too far away, like in another galaxy, measurements can be performed each 6 months, when the earth is at opposite places in its orbit around the sun. The width of earth’s orbit is good enough to use parallax in order to make out distances light years away.

All the above is IIRC though, it’s been ages since I delved into that stuff. You might want to read further on the subject of parallax in astronomy to make sure your plot is accurate.