The cosmos is a mysterious place, but we know a surprising lot about it. The knowledge, known as astronomy’, has been primarily acquired through observing the skies night after night, year after year. People used their eyes as the instrument of observation for a long time, but the problem is that the opening of the eye, called the pupil, is just too small – about one-eighths of an inch across. Very little light gets through and thus dim cosmic objects are invisible to the naked eye. For progress, we needed bigger apertures with which we could collect light and something which could automatically record images of the night sky. It turns out that a bigger aperture emerged long before the recording instruments.
One facet: ‘We need a bigger aperture’
A telescope is just a bigger and simpler version of the eye. Light falls on an opening, called the aperture, and is then focused onto a smaller opening, called the eye-piece, for a person to look through. Telescopes are basically of two types the reflecting ones and the refracting ones.
The main aim is to maximize the aperture for greater collection of light. This is easier in case of reflecting telescopes than for refracting telescopes. The problem is that a bigger aperture needs a bigger lens. Lenses tend to get heavy as they become bigger. Not only that, they tend to sag under their own weights. For a reflecting telescope, a bigger aperture does require a bigger reflecting mirror, but that is much less of a problem. This is the reason why reflecting telescopes have taken over. All large optical telescopes today are reflecting ones.
Examples of really large modern optical telescopes are the KECK and the SALT. The most successful of all telescopes is, of course, the Hubble Space Telescope.
Another facet: ‘We need a large telescope in space’
The fact that a lot of radiation doesn’t reach the earth’s surface is a major stumbling block to observations in wavelength regions in the electromagnetic spectrum, other than optical. The following image explains this phenomenon the best. Note the wavelength regions that do reach and those that don’t.
Thus, it is impossible to build a ground based gamma or x-ray telescope as long as we have an atmosphere. The only option is to send the telescope off into space. The Chandra X-Ray Telescope is the best of the x-ray telescopes.
The atmosphere creates problems even for the optical band of radiation. Air movements and thermal effects create density gradients both locally and globally in the atmosphere. These distort the passage of optical light – an aberration called ‘seeing’. Hence the Hubble Space Telescope was a necessity rather than a luxury, and how it has proved itself!
Radio is a region where a lot of observation has been done. Radio Telescopes are easy to build and radio is not attenuated by the atmosphere. By using certain specialized techniques, very accurate information can be found. Often cosmic structures have regions where the gas and dust density is so high that optical and even X-rays are absorbed. These optically opaque regions are conveniently observed in the radio band.
Advanced imaging techniques have greatly improved on the amount of information we can obtain from an observation. Advances in solid state physics have allowed us to create excellent image reproduction devices like CCD’s and CMOS. One thing is for sure once man had looked up and seen the cosmic wonders, he was sure to get addicted to it.