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

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. Commented Apr 23, 2021 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?") Commented Apr 23, 2021 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? Commented Apr 27, 2021 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
Commented Apr 27, 2021 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. Commented Apr 27, 2021 at 9:32

Partial answer, here is one notable example.

Why does X-shooter use double passes through prisms for Echelle cross-dispersion instead of gratings? informs us that prisms are heavily used in this amazing instruments. (related: What are the pros and cons of different types of echelle spectrograph cross-dispersers?)

1. Are there any notable cases or observations where this is/was very important?

Yes! X-shooter:

The concept of X-shooter has been defined with one principle goal in mind: the highest possible throughput over the wavelength range from the atmospheric cutoff to the near infrared at a resolution where the instrument is sky limited in a half hour exposure.

The option to use a single slit in the telescope focal plane was rejected because of the difficulty of designing a highly efficient relay system and atmospheric dispersion correction for the full spectral range...

The optical design allows the introduction of two short-wavelength atmospheric dispersion correctors (ADC) and the focusing of the target on the slit units at the entrance of the respective arms.

Interestingly as the telescope tracks near one wavelength **it is necessary to also correct tracking for atmospheric dispersion as well:

For slit observations (but not IFU) these tip-tilt mirrors also compensate for shifts due to atmospheric differential refraction between the telescope tracking wavelength (fixed at 470 nm) and the undeviated wavelength of the two atmospheric dispersion correctors (for UVB and VIS arms, see Sect. 2.2.7) and the middle of the atmospheric dispersion range for the NIR arm.

Here is the bulk of the description:

2.2.7. The focal reducer and atmospheric dispersion correctors

Both UVB and VIS pre-slit arms contain a focal reducer and an atmospheric dispersion corrector (ADC). These focal reducer-ADCs consist of two doublets cemented onto two counter rotating double prisms. The focal reducers bring the focal ratio from f/13.41 to f/6.5 and provide a measured plate scale at the entrance slit of the spectrographs of 3.91′′/mm in the UVB and 3.82′′/mm in the VIS. The ADCs compensate for atmospheric dispersion in order to minimize slit losses and allow orienting the slit to any position angle on the sky up to a zenith distance of 60°. The zero deviation wavelengths are 405 and 633 nm for the UVB and the VIS ADCs, respectively. During slit observations, their positions are updated every 60 s based on information taken from the telescope database.

Since the IFU comes ahead of the ADCs in the optical train, no correction for atmospheric dispersion is available for IFU observations, and the ADCs are set to their neutral position in this observing mode.

The NIR arm is not equipped with an ADC. The NIR arm tip-tilt mirror compensates for atmospheric refraction between the telescope tracking wavelength (470 nm) and 1310 nm which corresponds to the middle of the atmospheric dispersion range for the NIR arm. This means that this wavelength is kept at the center of the NIR slit. At a zenith distance of 60° the length of the spectrum dispersed by the atmosphere is 0.35′′, so the extremes of the spectrum can be displaced with respect to the center of the slit by up to 0.175′′.

I can't find a separate image of the dispersion correction prism, probably because it is placed before the entrance slits of the spectrometers.