Wednesday, October 07, 2009

2009 Nobel Prize for Physics Part 2: Fiber Optics

Fiber optic cables used in McDonald Observatory's HETDEX project
Image Credit: HETDEX / McDonald Observatory

As I mentioned yesterday, this year's Nobel Prize in Physics was shared between scientists who developed digital imaging circuits known as Charge-Coupled Devices (CCDs) and a scientist, Dr. Charles Kao, who designed the first fiber optic cables that were useful for long-distance data communication.  Yesterday I blogged about the astronomy uses of CCDs, so today I'll talk about the astronomy uses of fiber optic cables.

At most telescopes, there are two primary kinds of instruments.  One kind is the imaging camera, which simply takes pictures of the sky.  That's easy enough to understand, and fairly straightforward to build.  (Don't get me wrong, building any astronomical instrument for a big telescope is very hard.)  The other type of instrument is a spectrograph, which splits light into its component colors.  These spectra are most often used for determining the chemical composition of things and for measuring how fast things are moving.

One problem with spectrographs in the past has that the number of stars or galaxies you can look at at one time is limited.  Some spectrographs only allow you to look at one star (which is fine if there's only one star in some area of the sky that you are interested in),  Some allow you to look at multiple stars or galaxies, but which ones you can look at are constrained by geometry -- you can't analyze spectra of individual objects if they criss-cross or lie on top of each other.  And if you are looking at a two-dimensional object, like a galaxy or nebula, traditional spectral only allow you to get a  spectrum of  a long thin slice of the object.  So, how can we get around these problems?

One solution to this problem has been fiber optics.  With a fiber optic spectrograph, astronomers place fiber optic cables over each of the stars or galaxies they are interested in.  These fibers are then rearranged in the camera so that the resulting spectra do not overlap.  It's very clever, and it's allowed astronomers to take spectra of hundreds of objects at once (Sean at Cosmic Variance blogged yesterday about how this capability allowed the Sloan Digital Sky Survey to survey a large chunk of the nearby Universe).  Or, if the astronomer wants to take a spectrum of an entire galaxy, she can just squeeze all on the fibers into a big bundle, so that light from any part of the galaxy will land on one of the fibers, giving us a spectrum of everything. The only trick is remembering which spectrum belongs to which fiber and where that fiber was on the sky, so we have to use careful documentation and computer controls to keep track of those things.

The McDonald Observatory is currently building a giant fiber-optic instrument called VIRUS as part of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX).  VIRUS will use 34,000 fiber optic cables bundled into 150 bunches like the one pictured at the top of this article.  In a single pointing of the telescope, VIRUS will take a spectrum of everything in a region of the sky a bit smaller than one tenth the area of the full moon.  Over the entire HETDEX experiment, several thousand of these exposures will be taken, adding up to an area of sky roughly as big as 500 full moons!  This experiment would be impossible without fiber optics.

Fiber optics are also used to take light from the telescope and send it into an instrument housed in a separate room.  Instruments are usually mounted on the telescope itself, and so they get jostled and turned (sometimes upside-down!) as the telescope follows objects across the sky, and the instrument can even change size slightly as the air around it cools off during the night.  With careful engineering, those jostles and rotations and thermal contractions can be minimized so that the camera is quite useful.  But some science, like looking for Earth-sized planets around other stars, requires a much more stable instrument.  So, with fiber optics, we can build the instrument in a temperature controlled room that is isolated from the bumps and wiggles of the telescope, and send the light from the star to that room.  Such instruments are among the most precise astronomical tools now in use, and were used to measure the tug of a planet only twice the diameter of the Earth on its parent star.

So, in short, modern astronomy owes a lot to all of this year's Nobel Prize winners.  Congratulations and thanks to Drs. Kao, Boyle, and Smith!

1 comment:

  1. Anonymous5:42 AM

    Thank You Professor, this was very informative.