The Two 16 Towers

Contributed by
Oct 19, 2005

This doesn't look much like a telescope, does it?

But it is! Sure, it looks like a bunch of steamer trunks lashed together, but it's actually the imaginatively-named "Large Area Telescope", or LAT, the main instrument that will be part of the Gamma-Ray Large Area Space Telescope (GLAST), a major NASA mission due to launch in 2007. GLAST is designed to observe high-energy gamma rays, which are just like the visible light photons we see with our eyes, but much more energetic.

As I mentioned in this blog entry from Monday, the kind of light an object emits depends on the energy of the object. Gamma rays are very high energy (millions or billions of times the energy of optical light), so it takes a pretty serious event to make them. For example, exploding stars, superdense neutron stars with magnetic fields a quadrillion times the strength of the Earth's, tortured matter heated to millions of degrees and focused into tightly-collimated beams as it screams away from a black hole-- these are the kinds of things I'm talking about.

Up until now, we've had only a fuzzy view of these objects. That's because, unlike visible light, it's nearly impossible to focus gamma rays. They don't act like regular photons: they'll pass right through the glass or metal in a mirror, for example. So you need to have some exotic designs for your detector to be able to figure out where any given gamma ray comes from.

The LAT can do it. It uses those 16 towers in the picture. Wanna know how? More importantly: are you ready for this?

Each tower consists of a stacked series of layers-- 36 of them. Each layer has a series of interleaved silicon strips, sort of like wicker. When a gamma ray photon smacks into the tower material, it creates an electron-positron pair. These travel down to the bottom of the tower and into a device called a calorimeter, which is used to determine how much energy the pair of subatomic particles has, which in turn tells the LAT how much energy the original gamma ray had. The position on the sky (what part of the sky it came from) of the incoming gamma ray is determined by back-tracking the path of the electron-positron pair (like looking at skid marks on the road to see where a car came from before it crashed, or tracing footprints in the sand) through the layers. The LAT is accurate enough to pin down the gamma ray location to about 0.5 arcminutes in the sky (for comparison, the Moon is 30 arcminutes across). That's a lot better than any gamma-ray observatory previous to GLAST.

The process is actually hugely more complicated than this, of course. The LAT was designed at Stanford, where lots of smart people hang out and build things like this, so it's bound to be complex.

Below is a schematic of a LAT tower. When all 16 are put together like in the picture above, the assembly is a couple of meters across. It's pretty big.

When it gets into space (Earth's air absorbs gamma rays, so you need to get up above the atmosphere), the LAT will be the Hubble of gamma rays. We'll have better views of monstrous black holes gobbling down matter, viciously spinning pulsars blasting out energy, and supernovae explosions which generate gamma rays as they ferociously accelerate electrons in their magnetics fields even as they alchemically brew the elements in our blood and bones.

I'll add that the majority of my funding at work for the past five years is from GLAST, so it's pretty cool to be able to finally write something about hardware getting built (and being part of the team is how I got access to that picture of the towers, which was released just earlier today!). When you're building a new observatory, it takes a long time before you can actually start cutting metal. It's great to see that this fine observatory is finally on its way!

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