Tuesday, October 06, 2009

And the Nobel Prize for Physics Gadgets With Huge Astronomy Impact Goes To...

Many people will be blogging today about today's announcement of the 2009 Nobel Prize in Physics, what the awards are for, who did what, and who didn't get recognized that should have.  so I thought instead I'd focus on the astronomy aspects of today's awards, which are very important.  First, one paragraph on today's awards.

This morning, the winners of the 2009 Nobel Prize in Physics were announcedDr. Charles Kao won half of the prize for making some tremendous contributions in fiber optics that led to their usefulness for communication. (Contrary to some reports I've read, Dr. Kao did not invent fiber optics; he and his collaborators found a means to allow fiber optics to send messages over long distances, necessary if you want to make a phone call via fiber optic cable across a continent or ocean.)  Dr. Willard Boyle and Dr. George Smith jointly received the other half of the award for their work on charge-coupled devices (CCD), which are one of the major types of digital cameras.  Most cheaper digital cameras, including, most likely, any that you own, use a different type of digital imager called CMOS (read here to learn about the difference), but the CCD has traditionally provided better images and better sensitivity.

Both of these achievements have had crucial impacts on astronomy.  Today, below the jump, I'll talk about the CCDs, and tomorrow I'll talk about fiber optics.

The CCD is an electronic device that works on a principle for which Einstein won a Nobel Prize in 1921 called the photoelectric effect.  When light hits some kinds of metal (like silicon), the light is absorbed and an electron, that tiny electrically-charged particle that is the basis of electricity, is released.  Einstein's award came because he realized that the photoelectric effect could be explained by light acting as a particle (which we call a photon).  An electron in an atom of silicon can absorb a single photon and be freed from its bounds to the atom.  Add a little circuitry, and you can even make an electric circuit.  This is the basis for photovotaic cells (solar panels that produce electricity).

Another thing that Einstein realized was that the number of electrons released was equal to the number of photons absorbed.  In other words, if you could count how many electrons were released by a piece of metal, you know how many photons hit that piece of metal and were absorbed. 

A CCD is, at its simplest, a device that has a bunch of little pieces of metal (usually silicon) arrayed in a grid.  An image can be focused using mirrors or lenses on this grid, and each of the little pieces of metal (let's call them picture elements, or pixels for short) counts how many photons hits it.  Then some more circuitry counts the number of photons in each pixel, and you can reconstruct the image.  If you then make a little computer file that keeps a record of how many photons were in each pixel, you can reconstruct the image on your computer months or years later.  And you can send that information (by radio waves, by the internet, even by Morse Code through fiber optic cables) to anybody else, so everyone can see your image.  In other words, a CCD can be used as a digital camera.  Neat!

In astronomy, CCDs have replaced photographic film as the primary way of detecting light from astronomical objects.  The reason for this is pretty simple.  Since we can count how many photons came in from a star, planet, nebula, or galaxy, we have a better record of how bright that object is.  We can also be clever by using filters or prisms to split the light into the colors of the rainbow, and then we can count how many photons we get in different colors of light.  Photographic plates also record how much light you get, and you can play the same tricks with filters and prisms, but they don't provide a hard number count of the number of photons.  In physics and astronomy, the best science requires hard numbers.   CCDs win.

CCDs also took over because they provided digital information that could be stored on computer disks and easily duplicated and shared with collaborators and with the world.  Photographic plates can be duplicated, but it is a hard process to do that without losing information (ever see a photocopy of a fax of a photocopy?).

Another big advantage of CCDs over other types of imagers, including other types of digital imagers, is that CCDs are very efficient.  A well-made CCD can turn 95% or even more of the photons that hit it into electrons, and they can have a pretty uniform efficiency over the entire camera.  Even more important, the efficiency is repeatable, so even if one corner of the camera doesn't detect light as much, you can use calibrations to calculate exactly how efficient each pixel is, and so you can correct for efficiency differences.  The sensitivity of photographic film can vary from one batch to the next, and even from one frame to the next. 

For these reasons and others, CCDs are the workhorse instrument of astronomy.  If you go to any opticla observatory, look inside the Hubble's cameras, or look into X-ray and ultraviolet telescopes in space, you'll find a CCD at the heart of the vast majority of cameras.  Those cameras that use other detectors usually have special design needs.

There are some important places in astronomy where CCDs do not rule.  These include the highest energy light (gamma rays) and the lowest energy light (radio waves, microwaves, and much infrared light).  At the high energy end, silicon is transparent.  The gamma ray photons are so energetic they just pass right through silicon without leaving a trace.  For radio waves and other low-energy types of light, the energy in a single photon of light is not enough to free the electron from its parent atom, and so the photoelectric effect doesn't work.  In the infrared, there are imagers related to CCDs that can use special materials that still use the photoelectric effect, but in microwaves and radio waves the individual photons are just far too puny.  At those wavelengths of light, we use antennas to detect the photons' wave-like nature.

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