The Two 16 Towers

By Phil Plait | October 19, 2005 9:15 pm

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!

CATEGORIZED UNDER: Cool stuff
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Comments (8)

  1. Thomas Siefert

    If I understand this correctly, it’s like tracing the path of photon through the glass of a lens and then computing the image from that?

    It looks like an acoustic tower at a rock concert; bet you guys got a volume knob that goes to 11.
    Rock ‘n’ Roll!!!

  2. The LAT is accurate enough to pin down the gamma ray location to a few arcminutes in the sky (for comparison, the Moon is 30 arcminutes across).

    Does that mean that the LAT can “focus” to within roughly 1/30th of the diameter of the moon? I.e., that at best it can take a 30 pixel by 30 pixel photo of the moon? If so, that doesn’t sound very impressive, and nigh-useless for taking pictures of more distant objects, so I hope I’m misunderstanding something.

    Or does the LAT use results from all 16 towers to improve accuracy and focus better? Or is the nature of gamma rays such that even a 30×30 image of the moon is better than astronomers have been able to do so far?

  3. pumpkinpie

    Thanks for the article! I’m interested in finding out how exactly telescopes work to focus the different kinds of energy and make pics, so now I can cross Gamma Rays off my list!

    When you describe the towers, you say “a series of interleaved silicon strips, sort of like wicker.” Is interleaved a term unfamiliar term to me that I’ll have to look up, or did you mean interweaved? (wicker reference.)

  4. I corrected the resolution; it’s actually 0.5 arcmin (I had read an old document yesterday when I wrote “a few arcminutes”). EGRET, the best gamma-ray observatory before GLAST, had 15 arcmin resolution! So this is 30 times better. It sounds bad compared to optical ‘scopes, but focusing gamma rays is nearly impossible (which is why most detectors don’t try to focus). It’s possible that in 10 years we’ll have better tech, but for now GLAST is the best we can do.

  5. “Interleaved” is similar (if not the same as) interweaved.

  6. Holy flying bat guano, Batman! The GLAST observatory detects gamma rays via pair production?!

    That means it can only detect gamma rays of at least 1 MeV, right? I mean, lower-energy gamma rays won’t have enough oomph to create a positron and an electron. Does GLAST have some other means of detecing those ho-hum garden-variety gamma rays in the mere 100-900 keV range?

  7. Tracer, you’re correct. But lower energies are more like hard X-rays. :-)

    There is another instrument called the GLAST Burst Monitor (to look for GRBs) that is sensitive to lower energies, down to a few keV.

  8. 1plus8, a relatively well known youtuber, claims that because of the gamma rays produced by the moons interaction with cosmic rays would make a landing fatally hazardous. I want to hear your opinion on this specific claim, if it isn’t to much trouble. I have as yet found little to rebut him on this specific issue.

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