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Atmospheric refraction (shown below) happens because Earth's atmosphere has an index of refraction that differs from unity.

@MikeG's comment mentions that this refraction would have a chromatic component (since the index of air varies with wavelength) and that observers sometimes use a wedge prism to correct for it.

I suppose it would be more important for a wide spectrum image than for narrow band imaging.

  1. How often is this done in practice these days?
  2. How often was this done in the past with emulsion rather than CCDs?
  3. Are there any notable cases or observations where this is/was very important?
  4. Roughly how strong is the effect? If the average refraction is 2 arcminutes, roughly how many arcminutes would a glass wedge need to be to correct the chromatic aberation of the atmosphere?

Plot of atmospheric refraction vs. apparent altitude, using G.G. Bennett’s 1982 formula. Author: Jeff Conrad

enter image description here

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