# When are wedge prisms used to correct chromatic effects of atmospheric refraction?

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

• I just learned, that so-called Atmospheric Dispersion Correctors (ADC) are advertised on various websites for prices of around 0.2k\$, so ADC might be a helpful keyword here. – B--rian Apr 23 at 8:21
• The ESO also has a page on the maths of atmospheric dispersion correction which I found interesting. If you know German (or how to use your favorite online translator), there is also Wie stark ist die Unschärfe durch Dispersion? ("How much blurring is caused by dispersion?") – B--rian Apr 23 at 8:22
• I do not understand what you mean with "How often was this done in the past with emulsion rather than CCDs?". Don't you mean ADC instead of CCD? CCD is the sensor, and directing light through an emulsion with a refractive index gradiant would be a solution to correct for the refractive index gradiant of the atmosphere at dusk or dawn. Or what kind of emulsion are you talking about? – B--rian Apr 27 at 8:11
• @B--rian I think it's a question of how old we are. Photographic emulsion = "film" was used before CCDs were invented (e.g. photographic plates). Remember film cameras? Kodachrome or Fujichrome? Taking your snapshots as a roll of film to the store and getting your pictures back 3-5 days later? – uhoh Apr 27 at 9:21
• Ups, now I see. I worked with photographic film as well (not in astronomy though), but I have not been acquainted to the term emulsion. – B--rian Apr 27 at 9:32