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Infrared in the spotlight (1)

07/11/2002

THE NEXT MAJOR DISCOVERIES ABOUT THE UNIVERSE WILL ALMOST CERTAINLY BE MADE USING INFRARED. THE GRAN TELESCOPIO CANARIAS (GTC) IS GETTING READY: EMIR, ONE OF ITS INSTRUMENTS, WILL BE UP TO THE TECHNOLOGICAL CHALLENGE.

The range of light that can be seen with the naked eye is so limited that we miss a lot of detail. We need to be able to see more if we want to understand what lies behind the mysteries of the Universe. Throughout our history invisible phenomena have spawned new technologies: radio waves, X rays, infrared...These are all types of light - light we cannot see.

To look but not see: this is the concern of scientists, who suspect that up there amongst the stars, where our eyes see only blackness, there are objects that can tell us, molecule by molecule, about what and when we came from - the explosion which, in a mass of chemical reactions, created what we know today as life.

It happened millions of years ago and lasted only tenths of a second: Big Bang was a tremendous explosion that caused gases, fire and matter to expand and, according to the theory, created everything we can see today with our eyes and our instruments. So what else is up there? And, the most interesting question: can we see it all?

The power of the invisible

There are other types of ‘light’ that we cannot see but that we can detect and analyse using the right kind of instruments. Two centuries ago we had only the evidence of our own eyes to find out what kind of light existed. When electromagnetic radiation was discovered, made up of photons operating at different wavelengths beyond our vision, it revolutionised the laws of physics.

Radio waves were discovered and used for communication; microwaves were put to use in radar systems and domestic appliances; ultraviolet light was found to stimulate the skin, producing melanin; X rays were harnessed in the service of medicine; gamma rays were discovered and found to be waves of energy, damaging to human tissue like X rays and ultraviolet light; and infrared, which exists as heat, was discovered.

Infrared rays are commonly used in our everyday lives: we turn on the television and change channels by remote control; at the supermarket the bar codes on the products we buy are scanned; we watch and listen to compact discs...all thanks to infrared. These are just some of the simpler applications - infrared is also used for security systems, marine research, medicine and much more.

Infrared is very useful in astrophysics. It is a type of ‘invisible light,’ and carries valuable information with it about the objects we want to study - for example, it can tell us what chemical compounds they are made of. Some objects emit most of their energy in these ‘invisible’ light ranges and so they remained hidden until this ‘new light’ was discovered. This is particularly true of very distant objects and those that are hidden behind huge clouds of dust, which block out visible radiation, but not infrared.

Infrared...

EMIR is the name of an ambitious instrument that will be installed at the Gran Telescopio Canarias (GTC) in approximately 2006. It is designed to ‘see’, in the infrared spectrum, objects that are difficult to detect in the visible range (the range that can be seen with the naked eye). With EMIR we will observe faint galaxies, low-mass stars, young stellar objects, brown dwarfs…any object that can only be observed in the infrared either because it lacks the thermonuclear reactions normally found in stars, is weak, or is hidden behind clouds of gas and dust.

An interesting phenomenon also has to be taken into account: light shift, or the Doppler Effect. In the spectrum of visible light, for example, longwave increases due to the distance of the light source and is pulled towards the red part of the electromagnetic spectrum. This effect, known as ‘redshift,’ means we have to use infrared to observe objects that we could otherwise observe in the visible light range if they were not so far away.

Instruments for large telescopes

The Instituto de Astrofísica de Canarias (IAC) is leading the team developing and building this second generation GTC instrument. To get to know EMIR better we discuss here its most important component, the eye of the instrument: the detector.

If we compare the optical components of a camera, which control the amount of light that enters, with those parts of the eye that perform the same function, the film or detector would be equivalent to the retina: the part that collects light and analyses it. In the human eye, light passes through the cornea, the iris, the pupil and the lens to reach the retina, where it is converted into nerve impulses that are understood by the brain. Light is processed by special cells known as rods (which detect shape and movement) and cones (which perceive colour).

A detector is similar to the retina. It is a device that uses a light-sensitive material that gives off electrons when it comes into contact with photons (particles of light) and stores them in the “electron pool.” We then convert them into electrical signals that we can interpret and analyse.

These electronic devices use light-sensitive cells called pixels: the size of each pixel is around ten thousandths of a millimetre. For work in the visible light spectrum, the most commonly used detectors are of the CCD (Charge Coupled Device) type, in which each pixel transfers electrons to the next until, one after another in an orderly manner, the last pixel’s charge has been read by a signal amplifier located at the end of the detector. (For work in the infrared, each pixel carries its own mini-amplifier, which reads the stored charge directly, making the detector’s electronics much more complicated.) After this, the electricity is converted into digital information and sent to a computer. The image is then ready for analysis.

The next question is, what will this detector be made of? We will give you the answer in the next instalment of “Infrared in the spotlight.”

Natalia R. Zelman

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