Ours Is Bigger, Probably

Pretty soon, Europe will be blasting the heck out of a Chilean mountain so that they can build a great big telescope there. This extremely large telescope will be so large, in fact, that they'll call it the Extremely Large Telescope. At 40m across its extreme largeness is beyond dispute. However, some articles claim it will be the world's largest telescope, so here's a friendly reminder that (despite being 40m wide) compared with some other facilities, this will still only be an Extremely Little Telescope.

Nonetheless, other names considered included, "The Really Quite Enormous Telescope",
the "Too Big to Fail Telescope", and, of course, the "Telescope of Devastation".

Because 300m > 40m. I've counted.

Now, obviously, radio telescopes and optical telescopes  work very differently, and the challenge of building a 40m diameter mirror is quite different to building a 300m diameter metal bowl. But a telescope is a telescope, and calling the ELT the world's largest is a bit like calling Snowdon the tallest mountain - sure it is, in Wales. Omitting that qualifier makes the statement.... well, wrong.

The ELT will be the world's largest optical telescope. And it will be mindbogglingly awesome. But the more general question of "which is the world's largest telescope ?" is slightly more tricky - and much more interesting - to answer.

OK, Hubble isn't a giant telescope, but let's face it, it's pretty darn neat.
Sphinx model came from here. Anyone who responds with claims about mystical pyramid energy will be given a stern glare.

The diagram above shows some notable present and planned classical telescopes - big reflective (single-dish) mirrors that focus light onto a receiver. Several other very large optical telescopes are also planned (e.g. the Giant Magellan Telescope, which will consist of seven massive mirrors stuck together, and the aptly-named Thirty Metre Telescope, the American rival to Europe's ELT). There are also several other large ground-based telescopes already operating (e.g. the Very Large Telescope) and while JWST will be the successor to Hubble, it's worth pointing out that the now inoperable Herschel is currently the largest single mirror* ever flown in space at 3.5m diameter.

* Arguably. Keep reading.

Many radio telescopes work in the same way as optical telescopes, it's just that the light they collect is a at a longer wavelength. So there's no need to make their mirrors smooth and shiny - they don't care about visible (or optical) light, and the radio waves aren't affected by small bumps in the surface. That makes it quite a lot easier to build really enormous radio receivers.

In fact, the Arecibo reflector is pretty much transparent to visible light. To save weight (and, interestingly, also to let enough light through so that the plants underneath don't die), each metal panel has lots of little holes, and you can easily see through it from underneath (just like the protective grille in a microwave oven, the radio waves are too big to pass through the holes).

See more.

And that raises an important question - does the reflecting surface have to be solid and continuous ? If you insist that the surface has to be like, watertight - then the world's largest telescope is probably the 10.4m GTC in La Palma. And by that rather contrived definition, then yes, the ELT will indeed be the world's largest telescope.

A more reasonable interpretation would be that the surface must be continuous at the wavelength it's observing. In that sense, Arecibo is a clear winner, and has been for over 50 years. But all glory is fleeting, and in a few years time the massive FAST will dwarf even Arecibo. This 500m behemoth will have the extra complication of a deformable dish - thousands of cables will pull the spherical reflector into a paraboloid, allowing it to point at different parts of the sky. Whether this crazy scheme will actually work remains to be seen.

And yet, by taking a few more liberties, even FAST won't be the largest telescope. If you don't mind your reflector having some bloomin' great gaps in it, then the remarkably obscure Russian RATAN-600 claims the prize. This crazy instrument is almost like what you'd get if you built Arecibo without bothering to find a sinkhole for the reflector - basically it's a huge ring of reflectors, 576m across, which focus the radio waves onto a central receiver.

My professional opinion is that it's freakin' weird.

But we can't stop there. If we're allowing gaps, we may as well allow arrays of telescopes and not just single dishes. Then we can get very large telescopes indeed. The 27 antennas of the so-called Very Large Array form a telescope up to 22 miles (35 km) across using a technique called interferometry, and it's pretty common to connect multiple telescopes spread across the entire planet*. And yet those wonderful, crazy Russian scientists refuse to let a little thing like the size of the planet stand in their way...

* There's a price to be paid, of course - you can use multiple telescopes to get very high resolution, but you lose sensitivity by having so much empty space (especially to low density material).

I suppose it is fairly large.

Radio Astron takes interferometry to extremes. This is a 10m diameter Russian radio telescope... iinnnn sppppaaaaacce ! 10m is too large to launch a conventional reflector, so, like a giant umbrella, it was folded up during launch and unfolded once in space. Working in conjunction with ground-based telescopes, it can effectively form part of a telescope 350,000 km across. Given the existence of a telescope larger than the Earth, the question of the "world's largest telescope" seems a tad petty.

Because the world is not enough.

Then again, up until now we've been biased by assuming that telescopes have to collect light emitted by distant objects. This isn't always the case. Neutrinos are particles (so not even part of the EM spectrum at all) of almost zero mass that almost never interact with anything - most of them pass straight through the Earth unhindered. Building a camera to detect them isn't really an option. So neutrino telescopes work in a completely different way : they detect the light emitted on the (very) rare occasions when a neutrino does happen to interact with, say, a water molecule.

Yes, that's a boat. Yes, there was a missed opportunity for a Bond movie here.
The spheres house the sensors that will detect the light emitted by neutrinos colliding with water molecules (when the entire tank is filled). 

The Super-Kamiokande neutrino telescope seen above holds 50,000 cubic metres of water in a cylindrical tank about 40m tall and 40m across. For more "normal" telescopes, the area of the reflector determines how many photons of light you can detect. For neutrino telescopes, it's volume that matters, not area. Which makes them fundamentally different, so we can't really compare them to classical telescopes. That would be completely unfair, so let's do it anyway.

If we emptied all the water of Super-Kamiokande into Arecibo, it would only fill the reflector to a depth of about 8m. Assuming it somehow didn't just drain away, which it would.

Critics agreed that budget cuts to the remake of Goldenye were a mistake.

So, can radio astronomers point at laugh at all the other astronomer's puny instruments ? Possibly, but it would be inadvisable. For one thing, there's another, much bigger neutrino telescope : IceCube. Instead of building an enormous tank filled with liquid water, this is an even more audacious project using the ice of Antarctica. Buried over a mile below the surface are 86 boreholes each filled with 60 detectors, enclosing an entire cubic kilometre (1 billion cubic metres) of ice. That's a mass of 900 million tonnes - easily enough to crush Arecibo a like a puny little bug.

LIKE A BUG !!!

Still, although its mass is much, much greater, a mere cubic kilometre is paltry compared to the 350,000 km baseline of Radio Astron. For now, then, radio astronomers can laugh at other scientists as long as they're very very careful about it. Neutrino telescopes may have the most physical mass, but radio telescopes have the biggest reflectors and the largest diameters. They're likely to retain the prize for biggest mirrors for the foreseeable future, but another exotic type of astronomy has the crown for largest diameter firmly in its sights.

Gravitational waves are nothing less than ripples in space itself, produced by any moving object. These distortions should be detectable using a system of lasers and mirrors - roughly speaking, if a wave passes through, the length between the mirrors changes, altering the time it takes the laser to travel along its path. The path of the laser beam needs to be as long as possible since the changes are damnably small. Current detectors have laser paths a few kilometres in length.

LIGO detector in Louisiana.

Much to the disgust of other astronomers, none of these facilities have ever detected anything. In fact, it's the radio astronomers - yet again - who can point and laugh, since the only convincing evidence for gravitational waves comes indirectly from observations of binary pulsars (first measured, by the way, at Arecibo), whose orbital decay is in perfect agreement with the predictions of general relativity assuming they're emitting gravitational waves.

However, unless something is staggeringly, astonishingly wrong with the theory, given enough time it will be possible to directly detect gravitational waves. Fed up of other astronomers calling them "gravy waves", the GW community has hatched a foolproof plan to massively increase sensitivity and make absolutely sure if the pesky things are real or not. And that involves another giant space telescope - one that makes even Radio Astron look just a little bit pathetic. If it works.

The LISA gravitational wave observatory will use lasers to create a telescope ~5 million km across. This is a no-lose situation. If it works, we'll have a whole other way of studying the Universe. If it doesn't - I mean if the telescope itself functions correctly but fails to detect anything - it will be the greatest non-detection since the aether, and a scientific revolution is sure to follow. And, best of all, until it's built the rest of the astronomy community can continue to poke fun at the gravy wavers with all their oh-so-interesting noise measurements.


So, there you have it. The world's largest telescope is a tricky question to answer, and maybe even irrelevant. Already we have telescopes as big as or even larger than the world, in some sense, with more on the way. And that's just artificial telescopes. Arguably, even more exotic techniques like pulsar timing arrays and gravitational lensing use entire galaxies to form natural telescopes. But that's another story.