Seeing clearly through the atmosphere
Ground-based telescopes, operating in the visible or near-infrared, all suffer from a limiting resolution due to the thin layer of atmosphere between the Earth's surface and the vacuum of space. The atmosphere is made up of a variety of elements, roughly 78% nitrogen, 21% oxygen and less than 1% of argon and carbon dioxide found near the Earth's surface. The minor constituents are ozone, atomic oxygen, water vapour, sodium vapour, nitric oxide and trace elements. The atmosphere, or at least the most significant part which affects astronomical observations, is roughly 100 km thick. The atmosphere is constantly on the move and exhibits local random changes in density and hence refractive index. The effects of the atmosphere are that stars appear to twinkle, images are blurred and fuzzy (appearing to fluctuate in intensity) and closely spaced objects cannot be discerned. Here we discuss modern techniques (called adaptive optics) for eliminating almost all the degrading effects of the atmosphere. The images from a ground-based telescope using adaptive optics are of almost the same quality as would be achieved if the same telescope were outside the Earth's atmosphere.