Peering through the fog
The refractive index of air is also dependent on the wavelength of light passing through it, which can cause dispersion as different wavelengths of light refract different amounts (think the Dark Side of the Moon album cover). Some telescopes have a pair of rotating or sliding wedge prisms in front of the instruments, unimaginatively called Atmospheric Dispersion Correctors, to counteract this by generating an equal but opposite dispersion. 6 However, since the severity of both dispersion and refraction are strongly dependent on zenith angle, the most simple mitigation measure to atmospheric refraction is to observe only bodies that are high in the sky. Turbulence, seeing and scintillating Unfortunately, the problem of refraction is complicated by the fact that the Earth’s atmosphere is not entirely uniform. Local, random variations in pressure and temperature in the atmosphere can cause nearly unpredictable variations in its refractive index. 7 Ever been flying in an aeroplane and experienced the cabin shake at 35,000 ft? The same effect that causes an uncomfortable ride and a spike in anxiety also causes adverse effects to stargazing— turbulence causes the target star to appear to move and change intensity in an effect called seeing. 8 In relatively long exposure times this jittering leads to stars appearing as disks (larger than the disks produced by diffraction), which leads to a loss of data. There are several methods to reduce the impact of seeing: as it is driven by atmospheric turbulence, the selection of an observatory location which experiences stable airflow and is at a high altitude can reduce its intensity. In addition, since seeing is a result of the integral of the turbulence across the entire atmosphere, taking observations at low zenith angles also reduces the effect’s severity. Since the effect is random, a brute force approach can also sometimes work. ‘Lucky imaging’ works by taking many short exposures (<100ms), and then digitally recombining the exposures which were not badly affected by seeing, although this technique of course has the downside of increasing the required observation time; it also does not work with larger diameter telescopes. 9
Figure 3-Lucky Imaging (top 100%, 50%, 10% and 1%)
A relatively modern technique called adaptive optics can be a game-changer in reducing distortion: one of the telescope’s mirrors is distorted electronically (in ms timescales 10 ) to ensure that incoming wavefronts are aligned. The light from a guide star – either a bright, well-known star, or a laser designed
6 Parkerson S. ‘How an atmospheric dispersion corrector works’, https://astronomytechnologytoday.com/2017/07/06/atmospheric-dispersion-corrector/. Consulted: 26/08/2025. 7 Jones F. (1981) ‘The Refractivity of Air’, Journal of research of the National Bureau of Standards 86.1: 27– 32. 8 Bely, P. (2003) The Design and Construction of Large Optical Telescopes. New York. 9 Stephan, H et al. (2009) ‘The AstraLux Sur Lucky Imaging Instrument at the NTT’, The Messenger 137: 3. 10 Beuzit, J. et al. ‘SPHERE: the exoplanet imager for the very large telescope’, Astronomy & Astrophysics 631.
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