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Global Internet Telescope Tops Hubble's Resolution
satorchi writes "
The Arecibo Observatory
together with the
European VLBI Network have used the internet to make a
real-time transatlantic synthesis telescope. Data from the individual telescopes was transfered via the internet, and processed in real time by the central processing station at the Joint
Institute for VLBI in Europe. 9 terabits were transfered during the 20 hour experiment, and the resulting synthesised telescope had a resolution of 20 milliarcseconds, about 5 times better than the Hubble Space Telescope (HST). This level of detail is equivalent to picking out a small building on the surface of the Moon!"
9 terabits in 20 hours is slightly over 131 Mb/sec. Most of the telescopes were in Europe, but even assuming the Arecibo telescope generated three quarters of the traffic, 100 Mbit should be a drop in the bucket going across the Atlantic.
This method of merging data from multiple telescopes is equivilent to tiling together all the images from all the spectators at an event.
You get more information because of a larger number of eyes.
This principle has been known about for years and years, it just seems that the software/hardware to synchronise this and pull it off is coming into standard use.
From the article:
Until now, VLBI has been severely hampered because the data had to be recorded onto tape and then shipped to a central processing facility for analysis. Consequently, radio astronomers were unable to judge the success of their endeavours until weeks or months after the observations were made. The solution, to link the telescopes electronically in real-time, now enables them to analyse the data as it arrives. This technique, naturally called e-VLBI, is now possible as high-bandwidth network connectivity has become a reality.
liqbase
The comparison between Hubble's and Arecibo's resolution is misleading. The hubble telescoope operates in the viewable spectrum of light, while Arecibo and VLBI do radio astronomy. Radio waves are several magnitues longer, so it's even more difficult to get the same resolution. But since the frequencies are lower, too, it is possible to synchronize several telescopes using interferometry.
Interferometry is done at ESA's VLT with up to four telescopes and mirrors with a precision of about 10nm in the viewable spectrum, at a distance of about 100m. But here, we have a distance of several thousand kilometers, so the signals are digitalized and put together at the computer. This is difficult because it's really hard to synchronise the time -atom clocks are not precise enough. Hence the synchronisation is done "so that it fits best", not using any precise clock. (I don't think this is any easier to do, kudos to the scientists at arecibo and VLBI!)
Comparing a synthesised radio telescope (as was done here) with the Hubble is like comparing apples and oranges. It is MUCH harder to generate these kind of high-resolution pictures in visible as it is in radio.
For instance, if I were to use the VLBI technique in optical wavelengths, and if I had conditions where atmospheric turbulence wasn't affecting the image (as happens with radio), I would produce 20 milli arcsecond resolution with telescopes less than 10 metres apart, as opposed to telescopes on different continents!
Ground-based telescopes using adaptive optics surpassed Hubble years ago in terms of resolution. Prior to adaptive optics, atmospheric turbulance dictated a ground-based telescope's resolution (how close two objects can be and still be distinguished as "separate objects"). The advent of adaptive optics and telescope interferometry as largely solved the problem with the atmosphere so that resolution continues to increase with mirror size or in the case of multiple telescopes in an interferometer setup, the size of the baseline.
Ground-based telescopes have a number of clear advantages in addition to high resolution: they're easily upgraded/repaired and they cost far less than a Shuttle launch.
That said, space-based telescopes still have some advantages over their larger ground-based counterparts: first, they're obviously not subject to day and night but the big advantage is that a space telescope can observe in wavelengths that would be blocked by the Earth's atmosphere.
They're complimentary technologies.
The Hubble Space Telescope Project. This is an excellent guide to the 'scope and instrumentation on board the Hubble.
http://www.busyweather.com/
The principle involved is the same principle which has been used for some decades now in radio interferometry: the data (consisting of the electric field as a function of time) from several radio telescopes are recorded (with timestamps) and then sent to a correlator which combines the signals. This means that in contrary to optical interferometry, the interference is not realised in real time, but `simulated' afterwards in a computer.
The difference is in the way the signals are transported; they used to record the data on magnetic tape drives, which are still used but are more and more being replaced with hard disks. Apparently they have now also started to use the Internet to transport the data.
Very simply, this aperture synthesis experiment is not the same as being able to resolve a house on the moon, unless the house was emitting radio waves. Optical aperture synthesis is harder, but it has been done, at COAST, among others.
Owl tried to think of something wise to say, but couldn't.
Interferometric telescopes can drastically increase the resolution as compared to single-tube telescopes.
Having two scopes one mile apart, as far as resolution is concerned, is equivalent to having a single one-mile-wide mirror (in essence; the previous poster is correct in his argument about atmospheric distortions & other problems).
The problem is that the amount of light collected is still based solely on the sum of the surface areas of the mirrors-- not the effective area.
If not enough light (or radio waves, in this case) is collected to trigger the CCDs, the object throwing out the radiation simply won't be detected.
Incidentally, the Keck telescopes in Hawaii work this same way, but with a much shorter baseline. It helps that, at two miles above sea level, they're above much of the atmosphere, and that they both have fairly large mirrors to begin with.
For more information about how they work, Google lists plenty of resources.
Allegedly real newspaper headline from 1998:
Man Struck by Lightning Faces Battery Charge
Here's some math to explain what a resolution of 20 milliarcseconds really means.
:)), that is.
1 arcsecond = 1/3600 degrees
Therefore, 20 milliarcseconds = 20/3600000 degrees = (20/3600000)/360*2pi radians
Delta = arctan(diameter/distance)
where Delta stands for angular diameter. This formula is the basic definition of angular diameter. (Note : This formula automatically implies that the units of angular diameter are same as the unit of a plane angle, i.e. radian/degree)
Taking tan function on both sides we get
tan Delta = diameter/distance
Since resolution of the telescope is (20/3600000)/360*2pi radians we get
tan ((20/3600000)/360*2pi) = diameter/distance.
Now,
tan ((20/3600000)/360*2pi) = 9.69627362*10^-8,
This means that
9.69627362*10^-8 = minimum diameter/distance
which can be restated as
distance*9.69627362*10^-8 = minimum diameter
By substituting distance as required, we can obtain the diameter of the smallest observable object at that particular distance.
For example, taking (mean) earth-moon distance as 385,000 km we get
minimum diameter for an object on the moon to be observable = (385,000*9.69627362*10^-8) km = 0.0373306534 km = 37.3306534 m (approx.)
All math was done using Google's calculator and all formulae/definitions are from Wikipedia.
Disclaimer : I may have misinterpreted/misued the formulae so the above results are open to mistakes.
Mod this up anyway, I'm sure somebody will find my mistakes, if there are any (I hope not
Too bad it's a repeat of a repeat of a repeat of a repeat
It's superior to tiling. The content of the images is used to build a single composite image that eliminates much of the distortion received by any individual telescope because each telescope's distortion will be different. Every telescope can thus examine the same target, and produce a "picture" superior to what any one of them can produce.
"You're right," Fisheye says. "I should have set it on 'whip' or 'chop.'"
There will always be a need for space based telescopes.
A land based telescope maybe cheaper and have a higher resolution but it will always suffer from the effects of reflection, refraction, diffraction, absorption and scattering by the atmosphere.
The electromagnetic spectrum is huge compared to visible light or the small proportion that we can receive below the atmosphere.
The troposphere (less than 50km) contains mostly water, gas and pollution. At high frequencies (10GHz) rain and water vapor cause significant scattering. Above 25GHz the water and oxygen absorb much of the signal.
The ionosphere (greater than 50km) contains ionized oxygen. It acts like a mirror for low frequencies. It's mostly invisible above 50MHz. It is the reason low frequency radio can travel (bounce) around the world. However low frequency radio from space isn't visible on the ground. Let's not forget the huge amount of VHF/UHF noise created by television, radio, cell phones etc.
Using only the portion of the spectrum available on the surface to understand the galaxy/universe is the equivalent of trying to do science when all you can see and measure is a very narrow shade of red instead of all the colors.
Space based telescopes will always "see" more things.
(The previous post was "Plain Old Text" but unnoticed by me slashdot software interpreted the -less than- character and -greater than- characters as html - BUG. I also forgot to log in, the day is not shaping up well.)
- AndrewN