Differential Atmospheric Dispersion
Compute signed first-order atmospheric differential refraction from caller-supplied wavelength-dependent indices at actual conditions; near-horizon curvature is excluded.
Description
Wavelength-dependent refractive indices can produce differential atmospheric displacement.
Wavelength-dependent refractive indices can produce differential atmospheric displacement. A first-order tangent model gives a signed difference, preserving which wavelength was supplied first.1
Compute signed first-order atmospheric differential refraction from caller-supplied wavelength-dependent indices at actual conditions; near-horizon curvature is excluded.
When to use Differential Atmospheric Dispersion
Use this tool to compute signed first-order atmospheric differential refraction from caller-supplied wavelength-dependent indices at actual conditions; near-horizon curvature is excluded.
Prepare the source, detector or coordinate measurements. Confirm frame, epoch, angular units, axis directions and the stated RA proper-motion convention before entering values.
How to use Differential Atmospheric Dispersion and understand its results
- Prepare the source, detector or coordinate measurements. Confirm frame, epoch, angular units, axis directions and the stated RA proper-motion convention before entering values.
- Enter firstRefractiveIndex, secondRefractiveIndex, zenithDistanceDegrees using the field table and calculation rules, then run the tool.
- Read dispersionArcseconds. The explanation below defines the units, signs, ordering and any flags or placeholder values.
- If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.
| Input field | Meaning | Value type; see rules below | Requirement |
|---|---|---|---|
| firstRefractiveIndex | First refractive index | number | Required |
| secondRefractiveIndex | Second refractive index | number | Required |
| zenithDistanceDegrees | Zenith distance degrees | number | Required |
Enter indices each in [1,1.01] at the intended atmospheric conditions and zenith distance in [0,85] degrees. The tool evaluates the signed difference and converts radians to arcseconds. Equal indices or zenith distance zero yields zero dispersion.
dispersionArcsec=(n1−n2)tan(zenithDistance) × radiansToArcseconds Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.
| Output field | Meaning | Value type |
|---|---|---|
| dispersionArcseconds | Dispersion arcseconds | number |
{
"input": {
"firstRefractiveIndex": 1.0003,
"secondRefractiveIndex": 1.0002,
"zenithDistanceDegrees": 45
},
"output": {
"dispersionArcseconds": 20.62648062470736
}
}Limitations and assumptions
It does not calculate indices from wavelengths, weather or pressure. Near-horizon curvature, anomalous atmosphere and higher-order refraction are omitted; reverse input order reverses the sign.
Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.
Alternative or Complementary methods
Use Bennett Observed Altitude Refraction: Apply Bennett refraction to observed altitude, with caller pressure and temperature; anomalous atmospheric structure is not modeled. For a complementary task, use Kasten Young Optical Airmass: Evaluate Kasten–Young relative optical air mass for apparent altitude above the horizon; this is an atmospheric model approximation.
References
-
Differential Atmospheric Dispersion — tool.io
Similar or alternative tools
- Bennett Observed Altitude Refraction
Apply Bennett refraction to observed altitude, with caller pressure and temperature; anomalous atmospheric structure is not modeled.
- Kasten Young Optical Airmass
Evaluate Kasten–Young relative optical air mass for apparent altitude above the horizon; this is an atmospheric model approximation.