IR Atmospheric Windows

The Universe sends us light at all wavelengths of the electromagnetic spectrum. However, most of this light does not reach us at ground level here on Earth. Why? Because we have an atmosphere which blocks out many types of radiation while letting other types through. Fortunately for life on Earth, our atmosphere blocks out harmful, high energy radiation like X-rays, gamma rays and most of the ultraviolet rays. It also block out most infrared radiation, as well as very low energy radio waves. On the other hand, our atmosphere lets visible light, most radio waves, and small wavelength ranges of infrared light through, allowing astronomers to view the Universe at these wavelengths.

Most of the infrared light coming to us from the Universe is absorbed by water vapor and carbon dioxide in the Earth's atmosphere. Only in a few narrow wavelength ranges, can infrared light make it through (at least partially) to a ground based infrared telescope.

The Earth's atmosphere causes another problem for infrared astronomers. The atmosphere itself radiates strongly in the infrared, often putting out more infrared light than the object in space being observed. This atmospheric infrared emission peaks at a wavelength of about 10 microns (micron is short for a micrometer or one millionth of a meter).

So the best view of the infrared universe, from ground based telescopes, are at infrared wavelengths which can pass through the Earth's atmosphere and at which the atmosphere is dim in the infrared. Ground based infrared observatories are usually placed near the summit of high, dry mountains to get above as much of the atmosphere as possible. Even so, most infrared wavelengths are completely absorbed by the atmosphere and never make it to the ground.

From the table below, you can see that only a few of the infrared "windows" have both high sky transparency and low sky emission. These infrared windows are mainly at infrared wavelengths below 4 microns.

Infrared Windows in the Atmosphere

Sky Transparency
Sky Brightness
1.1 - 1.4 microns
low at night
1.5 - 1.8 microns
very low
2.0 - 2.4 microns
very low
3.0 - 4.0 microns
3.0 - 3.5 microns: fair
3.5 - 4.0 microns: high
4.6 - 5.0 microns
7.5 - 14.5 microns
8 - 9 microns and 10 -12 microns: fair
others: low
very high
17 - 40 microns
17 - 25 microns: Q
28 - 40 microns: Z
very low
very high
330 - 370 microns
very low

Basically, everything we have learned about the Universe comes from studying the light or electromagnetic radiation emitted by objects in space. To get a complete picture of the Universe, we need to see it in all of its light, at all wavelengths. This is why it is so important to send observatories into space, to get above our atmosphere which prevents so much of this valuable information from reaching us.

Since most infrared light is blocked by our atmosphere, infrared astronomers have placed instruments onboard, rockets, balloons, aircraft and space telescopes to view regions of the infrared which are not detectable from the ground. As a result, amazing discoveries about our Universe have been made and hundreds of thousands of new astronomical sources have been detected for the first time.

Due to the rapid development of better infrared detectors and the ability to place telescopes in space, the future is extremely bright for infrared astronomy.

Ground based infrared observatories, using advanced techniques such as Adaptive Optics are providing fascinating views of the infrared Universe viewed through our atmosphere's infrared windows.

Mauna Kea Observatories
Although these observatories cannot view at other infrared wavelengths, they can observe the near-infrared sky almost anytime the weather permits, providing valuable long term studies of objects in space.

New missions are being planned to get above the atmosphere to observe the infrared Universe with better resolution than ever before. SOFIA, an airborne observatory, is schedule to start operations in 2004. The Spitzer Space Telescope, launched in August 2003, is NASA's next great observatory in space.

In the next decade, you will probably hear much news about discoveries being made in infrared astronomy, as we now can see beyond our atmosphere's infrared windows!

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