Peering through the fog
especially good in this regard, 17 as the density of many absorbers decreases with altitude – especially water vapour concentration. Mauna Kei, with its altitude of 4,200m, is above 95% of the atmospheric water vapour, making it ideal for near-IR astronomy. 18 This is one of the reasons why the Keck observatory, Sub-millimetre array, Infrared telescope facility, James Clerk Maxwell telescope, Gemini North telescope and many more observatories call it home. 19
Figure 6 - The observatories at Mauna Kei
Atmospheric background The atmosphere does not just block out radiation; it creates it too, which can cause also cause problems for ground-based telescopes. Outside of the simple scattering of sun- or moonlight (minimized by observing during night and the darker phases of the moon), there are two mechanisms by which this occurs: fluorescence in the upper atmosphere and thermal emission. 20 The former occurs due to the recombination of atoms and molecules which have been ionized by sunlight, which results in the emission of photons of discrete wavelengths, mainly in the near-IR. Thermal emission is the same process which causes molten iron to glow or thermal cameras to work: bodies radiate photons, with a peak wavelength determined by Wien’s displacement law ( 𝜆 = where T is the temperature in kelvin and b is 2.897e-3 m K), 21 and that includes the gas that makes up our atmosphere. These effects combine to further limit the possible sensitivity of ground-based telescopes. Getting out of the atmosphere However, if astronomers want to avoid all these limitations and problems, there is an obvious other option: leave the atmosphere behind and put telescopes in space. The first successful orbital telescope was the Orbiting Astronomical Observatory 2 (OAO-2), which made nearly 23,000 measurements in the ultraviolet range between 1972 and 1981 22 – an unprecedented number given the atmosphere’s opacity to UV radiation. The most famous of all space telescopes, the HST, was launched in 1990 has been the subject of over 22,400 scientific papers over the last 35 years 23 – a testament to its success. It has also allowed for many discoveries such as the first confirmation that supermassive black holes 17 Perfetto, I. ‘The world’s highest observatory opens in Chile’, https://cosmosmagazine.com/space/astronomy/the-worldshighest-observatory-opens-in-chile/. Consulted: 26/08/2025. 18 Bely, P. (2003) The Design and Construction of Large Optical Telescopes. New York. 19 Maunakei Observatories. ‘Science’, https://www.maunakeaobservatories.org/science. Consulted: 26/08/2025. 20 Rafferty, J. ‘Airglow’, https://www.britannica.com/science/airglow. Consulted: 26/08/2025. 21 Kelly, G. (2015) Eduqas Physics for A Level Year 1 & AS . London. 22 Reddy, F. ‘NASA’s first stellar observatory, OAO 2, turns 50’, https://www.nasa.gov/solar-system/nasas-first- stellarobservatory-oao-2-turns-50/. Consulted: 26/08/2025. 23 See, for example, https://en.wikipedia.org/wiki/Hubble_Space_Telescope#Scientific_results.
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