ASTRONOMERS in each of these six sub-disciplines require a different type of telescope. Each instrument is sensitive to a particular bandwidth of electromagnetic radiation?wavelengths that carry distinctive information about both the object being observed and the space its radiation traverses.
Radio astronomy was the first of the five non-optical branches of the science to establish itself. Among its concerns are cosmic clouds, magnetic fields, star formation, the chemistry of the interstellar medium, radar studies of planets, meteoritics, galactic structure and the search for extraterrestrial intelligence.
Radio waves are longer?and thus pack less power?than any other form of electromagnetic energy. All the energy radio telescopes have ever detected on Earth, says Eric Chaisson and Steve McMillan in Astronomy Today, would barely keep a 100-watt bulb burning for 10 billionths of a second!
The instrument that collects this weak radiation must, therefore, be much more sensitive than an optical or other telescope. Ultimately, the cosmic radiation a telescope can detect from Earth depends on the wavelengths that pass through our planet?s atmospheric gasses?the atmospheric window.
According to the Collins Internet Linked Dictionary of Astronomy, this ranges roughly from about a millimeter to 30 metres for radio waves. Other wavelengths are either reflected back into space or absorbed in the ionosphere. (Hence gamma ray, x-ray and ultraviolet telescopes are deployed in space.)
As the science of astronomy evolves in Sub-Saharan Africa, the importance of radio telescopes is likely to increase exponentially. This is especially true in humid coastal areas, like southern Nigeria, where cloud cover and long rainy seasons render optical instruments 2can also penetrate clouds and be used when it is raining. That?s because most radio waves are larger than water droplets and atmospheric particles (which block optical photons) and, therefore, tend to loop around them.
?Optical astronomy cannot be done under these conditions,? notes Chisson and McMillan, ?because the wavelength of visible light is smaller than a rain drop, a snow flake, or even a minute water droplet in a cloud?.
Radio telescopes will add tremendously to the scope and depth of African astronomical research. For one thing, they enable investigators to study objects that are not visible to optical instruments?objects that radiate weakly at visible wavelengths but strongly in the radio spectrum.
Secondly, even objects that are visible optically will also radiate at other wavelengths, including radio. The radio telescope thus opens up a new dimension, adding depth and perspective to models based on optical observations.
The downside is that these instruments have very weak resolving power. They don?t distinguish between nearby objects well?and thus tend come up short on detail. The culprit is a phenomenon called ?diffraction,? which is the tendency of waves to bend around corners.
Here is where an interferometer comes in rather handy. It consists of two or more telescopes, usually separated by long distances, with cables or a radio frequency linking them to a central receiver where the cosmic radio signals are processed.
A radio interferometer behaves like a giant receiving bowl, whose diameter extends from the first to the last telescope in the system. It?s as if a humongous satellite dish spread from Nigeria to South Africa?although only one small dish in each country, is collecting radiation.
The result is a sharply focused radio profile of the object under study.
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