A Telescope as Big as the Earth

Roland Piquepaille writes "A week ago, seven telescopes around the world were linked together to watch a distant galaxy called 3C273 in real time and create a single world telescope. The data from these telescopes, which are located in Australia, China and Europe, was streamed around the world at a rate of 256 Mb per second. One of the Australian researchers involved in the project said that it was the first time that astronomers have been able to instantaneously connect telescopes half a world apart. He added that 'the diameter of the Earth is 12,750 km and the two most widely separated telescopes in our experiment were 12,304 km apart.'"

Comments

FYI

[Gabrill]

This technique is being used.

Re:FYI

[Bemopolis]

Total amount of signal gathered. That is to say, you gather less rain with a few buckets scattered across a field ratherthan a field-sized bucket. On the plus side, since you are monitoring simultaneously at different sites, you can compare the signals among the antennae to get the same spatial resolution of a telescope the size of the Earth. Compare to the VLA, a much smaller version of the same technique.

Re:FYI

[Nyeerrmm]

There are two reasons to make a really big telescope (whether optical, radio, or even x-ray). FIrst you pick up more photons, allowing you to pick up dimmer, more distant objects and get less noisy data. The second is that you get improved angular resolution, since the limiting factor for resolution on a good telescope is the diffraction of light, a consequence of the wave nature. Simply, the angular resolution is approximated very well by Rayleigh's Formula:

Resolution(radians) = Wavelength/Diameter

When you do this kind of technique, you increase the angular resolution that can be picked up to that of a full telescope over the area (if designed properly to get the middle resolutions as well). However, as others have mentioned, you don't get the full number of photons, which means you have to increase the imaging time or allow for much high SNRs. However, this is still very useful for getting high resolution images of fairly bright objects.

Re:FYI

[schwanerhill]

In addition to the lower light-gathering power, interferometers need to sample a wide range of separations between pairs of antennae. The most-separated pair of antennae (the longest baseline is the jargon) gets very small structures, but is not sensitive to larger structures. You get helped by the rotation of the Earth, which makes any given pair of antennae (which are fixed on the Earth) change their angular separation with respect to the target the array is looking at, but that only helps so much. You really need a range of separations, which means many antennae.

For example, the Very Large Array has 27 antennae. That's 351 pairs, which can be spaced differently. If you had a single dish telescope the size of the VLA (or the Earth), you'd get every angular scale at once, without having to synthesize a large aperture from all the baselines.

In practice, this aperture synthesis technique works quite well, and there's no way we're going to build a steerable single dish telescope larger than the Green Bank Telescope (100m in diameter) any time in the foreseeable future.

Re:Were they looking in the optical range?

[Robotbeat]

This is in the radio range, not the optical range. The summary misled me to thinking it was in the optical range, which would be an impressive achievement, indeed! The news of this story is that it was done in real-time, over a network connection, instead of by shipping data from each radio telescope site on hard-drives to a location be processed later.

Re:Cool

[Gloy]

You haven't tried Google Sky, then?

Re:OK

[evanbd]

It's a radio telescope; the atmosphere is almost irrelevant. This gives a very large effective size for diffraction purposes, meaning the resulting images can be much more finely resolved.

Re:Central Obstruction

[Liquidrage]

SCT's are Schmidt-Cassegrain telecopes.
http://en.wikipedia.org/wiki/Schmidt-Cassegrain_te lescope

They have a large central obstruction which houses the secondary mirror.

Central Obstrcutions come with negative affects.
http://www.telescope-optics.net/obstruction.htm

So I was making a very bad and geeky joke based on the headline about this being a very large telescope with the entire Earth as its central obsutrction. Which, in a *very* round-about sense, it is.

Re:Were they looking in the optical range?

[Nyeerrmm]

Just to expand on this comment for other readers, any time you do this with any kind of wavelength, you have to have the positions of the telescopes known within fractions of a wavelength. Radio waves range from meters to millimeters, so precision on a worldwide scale is difficult but not impossible at this range , although doing it in real-time is still an impressive feat, as this used to be done by recording the signals to tape, taking them to a central location and processing the data then.

However, expanding it to optical frequencies (where you can pick up different types of objects and also do so to much higher resolution) is difficult, since the wavelengths are around 500 nanometers, a level of precision that is still impossible on worldwide scales, except maybe in space, where you can depend on laser range finding over very long distances, although i don't know of any proposals trying to do this over very large scale.