Why Do Telescopes Sit on Mountains or in Orbit? What the Atmosphere Lets Through
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The atmosphere blocks most of the electromagnetic spectrum and admits two broad bands, one visible and one radio. Where the boundaries of those bands fall determines where every telescope is placed.
What gets through and what does not
Radiation arriving from space encounters an atmosphere that absorbs at wavelengths corresponding to what its molecules can do, and the resulting transmission varies enormously across the spectrum. Two broad regions pass relatively freely. The optical window covers visible light and extends somewhat into the ultraviolet and infrared on either side. The radio window covers a wide span of longer wavelengths, bounded at the short end by absorption from water vapour and oxygen and at the long end by the ionosphere reflecting signals back into space. Between and beyond those, ultraviolet, X-rays and gamma rays are absorbed almost completely, as is much of the infrared, which is why observing at those wavelengths requires leaving the atmosphere entirely.
What blocks each region
Different molecules are responsible at different wavelengths:
- •Ozone absorbs most ultraviolet, which protects life and prevents observation
- •Oxygen and nitrogen absorb shorter wavelengths still, including X-rays
- •Water vapour absorbs strongly across much of the infrared and at millimetre wavelengths
- •Carbon dioxide contributes to infrared absorption
- •The ionosphere reflects radio waves below a frequency that varies with solar activity
- •Dust and air turbulence degrade optical observation without absorbing it
Why telescopes go where they do
Site selection follows directly from that transmission. Optical observatories sit on high mountains to reduce the depth of atmosphere and the turbulence that blurs images, in dry locations because water vapour matters and because cloud does, far from cities because artificial light swamps faint objects, and frequently on islands or coastal ranges where airflow is smooth. Infrared and millimetre observatories go higher and drier still, which is why several sit above four thousand metres in the Atacama and one is at the South Pole. Radio observatories need protection from human transmissions rather than from weather, which is why they sit in designated quiet zones with legal restrictions on nearby radio use. And instruments for the blocked wavelengths go into orbit, which is expensive and is the only option.
The windows nobody knew about
Each time a new part of the spectrum became observable, the sky turned out to contain things nobody had predicted. Radio astronomy began accidentally in the 1930s when an engineer investigating interference found a signal from the centre of the galaxy, and it went on to discover quasars, pulsars and the microwave background left by the early universe, none of which had been anticipated. X-ray astronomy from rockets and satellites revealed accreting compact objects and the hot gas filling galaxy clusters. Gamma ray instruments launched to monitor nuclear tests detected bursts from distant galaxies. Infrared surveys found star formation hidden inside dust. The consistent lesson is that opening a new window reveals a population of objects invisible in every other one.
How the atmosphere is worked around
Several techniques recover part of what the air takes. Adaptive optics measures the distortion introduced by turbulence hundreds of times a second and deforms a mirror to cancel it, which restores much of the resolution a telescope would have in space and has transformed ground-based astronomy since the 1990s. Artificial guide stars, created by exciting sodium high in the atmosphere with a laser, supply a reference where no bright star is conveniently placed. Interferometry combines signals from separated instruments, achieving resolution set by their separation rather than by their size, which is standard in radio astronomy and increasingly used optically. Airborne observatories operate above most of the water vapour. And balloon-borne instruments reach most of the way to space cheaply.
The takeaway
The atmosphere admits a visible band and a radio band and absorbs almost everything else, with ozone taking ultraviolet, water vapour taking infrared and the ionosphere reflecting long radio waves. That is why optical observatories sit high and dry, radio observatories sit in legally protected quiet zones, and instruments for the blocked wavelengths must go into orbit.