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.
- How often is this done in practice these days?
- How often was this done in the past with emulsion rather than CCDs?
- Are there any notable cases or observations where this is/was very important?
- 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?