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Earth's Radio Telescopes Combining Forces
Slatterz writes "I own a basic 70mm telescope, which I'm sure Galileo would have given his right arm for in 1609. In fact, this year marks exactly 400 years since Galileo first pointed a telescope at the skies — discovering the moons of Jupiter and helping to prove that the universe doesn't revolve around us. As a mark of respect, the United Nations has declared 2009 the International Year of Astronomy. Official festivities kick off this week in Paris and, to help start the celebrations, 17 radio telescopes in Australia, Asia, Europe and the Americas will track three quasars using something called "real-time Very Long Baseline Interferometry" — basically creating hi-res images by combining their data to simulate a telescope as large as the Earth. Sounds cool."
Since the 1970s telescopes from all over the world (and even in Earth orbit) have been combined to perform Very Long Baseline Interferometry. Data received at each antenna is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. At that later time, the data is correlated with data from other antennas similarly recorded, to produce the resulting image. Using this method it is possible to synthesise an antenna that is effectively the size of the Earth. The large distances between the telescopes enable very high angular resolutions to be achieved, much greater in fact than in any other field of astronomy. At the highest frequencies, synthesised beams less than 1 milliarcsecond are possible.
Any life is made up of a single moment, the moment in which a man finds out, once and for all, who he is.
Dont get a crappy scope. it will simply discourage you.
go to orion at http://www.telescope.com/control/main/ and buy a 8" dobsonian.
you will see things that the guys that have the cheap crap cant.
you will also have a crapload more light gathering than any small lens telescope can hope to have, giving you better star views and even seeing color very well.
http://www.telescope.com/control/product/~category_id=dobsonians/~pcategory=telescopes/~product_id=08943 is a PERECT beginners telescope. it works fantastic and does not have the crapload of problems and poor viewing that anythign smaller would have.
Also if it can be bought from walmart or radio shack or even elder beerman, it's crap. do not buy it.
I have one of those and the 12" big brother to it. the 8" I loan out all the time to people interested in astronomy and they freak out when they look at saturn and see the rings seperated from the planet unlike a lesser scope can do.
the only drawback is a 8" scope can BLIND YOU if you observe the moon without filters.
Do not look at laser with remaining good eye.
For more information, the website for all of the events in the International Year of Astronomy is here. It really is amazing what you can see when you get away from light pollution.
Government - If you think the problems we create are bad, you should see our solutions!
My contribution to the IYA2009 is two fold.
First, I'll be speaking at least twice this year on astronomy to the public. Once at my local library, next week actually on the Winter Constellation, and then again this summer at the local Audubon Society on Binocular Astronomy. Places like these are hungry for smart people like us to talk to the public about our passion.
The second is that I've vowed to get out and do more public observing. This is where you setup your telescope in a busy place, like in a square downtown, and exhort the public to "Come see the Moon!" You can read about one of my adventures last year at http://notthepainter.com/2008/07/come-see-the-moon/ . You can even do outreach to your friends, I've auctioned off star parties at a charity auction, and I brought my telescope to Thanksgiving dinner!
The point is, this is the year that you, the astronomer, should try and make a difference. (Oh, and for those who think you need to be super experienced to do it, you don't, I've been doing this almost 2 years now, hmm, maybe 3, I've been having so much fun I forget.)
It's a pity few even realize what a problem light pollution is.
Indeed. Galileo made many of his observations from the city of Venice. Back then, you could still see the stars from a city center. Now, even the outer suburbs are pretty degraded.
If you care about changing this (and a lot can be done), join the International Dark-Sky Association.
There is some evidence to suggest that perpetual light messes with the hormone balance in women, potentially leading to breast cancer.
Radio sensors record the full waveform so you can cross-correlate the raw signal to create a large synthetic aperture. Almost all optical telescopes just record intensity without phase, so you cannot synthesize large aperture. A few telescope sites send optical over wave guides for analog correlation. This works for optical telescopes within a few hundred meters of each other.
Really, it all comes down to what's known as Fourier optics. In the far field from a distant source, the mutual coherence (cross-correlation) of the light is the 2-D Fourier transform of the original image, through the van Cittert-Zernike theorem. If you're at all familiar with FTs, the components further from the origin correspond to the highest frequencies. So a larger telescope (essentially an analog IFT in this way of thinking, simulating propogating the wave pattern back to the source) is able to cross-correlate the light between the two furthest points on the aperture, allowing you to take into account higher frequency ranges, and get higher resolution.(1)
Therefore, there are two reasons for increasing a telescopes size:
1. You collect more photons, improving signal quality
2. You increase the resolution as you're able to gather more components of the Fourier band. This is limited on Earth due to the seeing effects of the atmosphere, unless you start dealing with adaptive optics.
This kind of work, using multiple apertures to synthesize a much larger one, gets you the second result without much help on the first, but at a much cheaper price. By manually taking the cross-correlations(2), you can get a map of the Fourier field of the image and computationally compute the original image from it.
(1) This is also why blockages over parts of the aperture don't have an obvious effect.
(2) which is why this only works with radio, optical frequencies are much too high to record the actual waveform, so physical beam transmission is necessary over carefully controlled (to 10 nanometers) path lengths.
Yes, this aperture synthesis is done to increase resolution, not sensitivity. In fact, all known radio sources have a finite size for VLBI (because of the inverse compton effect), and so source brightness starts to decline as baselines go towards two Earth radii. While VLBI has been done from space, if we placed a VLBI antenna on (say) Mars, there would almost certainly be no source visible on Earth-Mars baselines for any obtainable antenna size.