Daily Archives: September 2, 2008

LSST, observatory mirrors

Where Do Telescope Mirrors Come From?

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Ovens…

Big Astronomy (and with it, the science of astrophysics) requires big mirrors to see farther and fainter across the universe.  If you visit such places as Palomar Observatory or Gemini, you see big mirrors that act as giant light buckets to capture as many photons as they can from dim, distant objects.  Mirror-building is getting to be quite an exact science, as we all found out from the Hubble Space Telescope, which has a mirror that was ground perfectly, exactly slightly wrong. (Precision works both ways, you know.) Since then, mirror builders have turned out ever more accurate and correct mirrors that are helping us see amazingly far out through the cosmos. Not only are they using single large mirrors, but some facilities are using arrays of smaller segments, joined together to create one large observing “surface.”  For this discussion, I’m going to focus on the single big mirrors because another big one has just come out of the hopper (or oven, actually) in Arizona.

How do they create these big ones?

When astronomers need a mirror, the first thing they do is have one made out of glass, which has to be cast in a furnace. If you’ve ever seen glassblowers at a street fair, it’s roughly the same principle in that they use molten glass to create their fantastic art.  In the case of a mirror, a huge amount of molten glass is created in a furnace and then cast into what is called a “mirror blank.”  That blank is then polished to high precision and to the correct curvature and put in the telescope. This, by the way, is pretty much the same way glass mirror blanks for small telescopes are made, too. If you ever go to a star party event such as Stellafane (held each summer in Vermont), you’ll see lots of people polishing their mirror blanks.

Fresh from the oven!

The latest “big” astronomy mirror to be cast is going into the Large Synoptic Survey Telescope at Cerro Pachón, Chile. It just came out of the ovens at the University of Arizona’s Steward Observatory Mirror Lab. It’s a 51,900-pound blank that contains a 27.5-foot (8.4-meter) primary mirror and a 16.5-foot (5-meter) tertiary mirror. Both were cast in the same mold, which is quite an accomplishment. This picture shows the blanks and quite possibly the last time that human hands will be allowed to touch the mirror. Now it will go in for polishing and shaping, before making the long journey south to its final installation.

The LSST will be the widest, fastest, deepest eye on the sky, aided by an incredible array of digital imaging instruments. If all goes well, science operations will begin in 2015 and astronomers will be able to do time-lapse digital imaging across the entire available night sky every three days. With this sort of large-scale mapping, they will be able to chart the structure of the universe and track events as they occur (such as supernova explosions). One of the most important science objectives for LSST is to explore the nature of dark matter and dark energy.

geodynamo, magnetic fields

Earth’s Magnetic Field

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How is It Formed?

That’s a very good question. The easy answer is that much of the magnetic field is formed as a result of electric currents generated deep beneath our feet. In reality, it’s much a much more complex set of processes that also includes a small amount of magnetism that already exists in rocks in Earth’s crust as well as magnetic fields generated from interactions with the solar wind. For this discussion, I’m going to focus on the action at the heart of our planet. This is where 97 percent of our magnetic field is generated and has been for the last 3.5 billion years.

The action that generates much of our magnetic field occurs in the liquid outer core that surrounds a solid inner core at the center of the planet. This diagram at left is a model of what geologists use to explain the process.

How does this process work?

A lot of times you’ll hear scientists refer to “Earth’s geodynamo” almost as if there’s a huge engine at the center of our planet that is generating the field. Actually, it’s a very nice analogy for what is a very complex set of actions. I’ll give you a “rough” outline here, since the actual mechanics are more complex than I want to get into here. However, if you’re interested in the complexities, visit the the United States Geological Survey Geomagnetism page, or the Comprehensive Modeling of the Magnetic Field page at Goddard Space Flight Center, or  When North Goes South, a page that explains how Earth’s magnetic field changes over time.

To generate and maintain a field such as Earth’s (or any of the planets, for that matter), you need electrically conducting fluids interacting with each other and with the existing magnetic field. Inside Earth, the fluid material is molten iron (and some nickel) that conducts electricity. You also need motion to create gentle currents (sort of like currents you see in the ocean) in the molten iron. That motion is supplied by the rotation of Earth’s various layers around our planet’s spin axis. The core rotates the fastest, the outer core rotates at a slightly slower rate, and the rest of Earth’s layers and the surface rotate even more slowly. The fluid iron sloshes around, adding to the electrical generation environment.

The currents flow across the already-existing magnetic field lines and this keeps the whole magnetic field-generating mechanism going. If the flow stopped (say if Earth stopped turning or somehow the fluid outer core was somehow cooled down to the point where it was no longer as fluid), the generating “dynamo” would shut down and the magnetic field would diminish over time. There would still be some “remnant” of the field emanating from rocks, but that would be it. Luckily, our geodynamo isn’t likely to shut off anytime soon, but if you want to see a planet that doesn’t seem to have a dynamo at its core, check out Mars. it does havve remnant magnetic fields from concentrations of magnetized rocks. But, there’s no overarching huge magnetic field being generated as there is here at Earth. That, in itself, is an interesting clue to Mars’s evolutionary past. Studying it could help us understand what happens to a planet when it loses its magnetic field.