3 Observing from the Earth
By now you will have had the chance to see for yourself the importance of waiting for your eyes to adapt to the dark and the benefits of getting away from light pollution. You may also have seen the effect that binoculars have on your ability to observe fainter objects and therefore to see more objects in the sky.
But these are not the only considerations in obtaining the very best view of the stars. The Earth’s atmosphere is an equally important part of the overall optical system from object to telescope to eye or camera. On its journey from a distant object such as the Orion nebula, light may travel for hundreds or thousands of year through empty space with very little disturbance, but on the last few tens of kilometres of its journey it has to pass through the Earth’s atmosphere on its way to your eye or your telescope.
Sometimes clouds in the atmosphere can prevent us from seeing the stars at all, but even on a clear night you have probably sometimes noticed the twinkling of the stars. This is the result of non-uniformities and turbulence in the air deflecting the light slightly. Through a telescope these small deflections are magnified, causing the image to appear to shimmer and move about. The atmosphere is also never completely transparent – dust and water vapour in the air can absorb some of the light, reducing the brightness and contrast of the image.
In order to avoid interference from light pollution and the disturbances of the atmosphere almost all professional telescopes are sited well away from populated areas and high up mountains or, in the case of COAST, on a volcano, as Miquel Serra-Ricart, administrator of the Teide Observatory explains.