Telluric contamination
Encyclopedia
Telluric contamination is contamination
of the astronomical spectra
by the Earth's atmosphere
.
s (electromagnetic waves) which originate beyond the sky. The molecules in the Earth's atmosphere, however, absorb
and emit their own light, especially in the visible and near-IR
portion of the spectrum
, and any ground-based observation is subject to contamination from these telluric (earth-originating) sources. Water vapor, oxygen, and OH are some of the more important molecules in telluric contamination. Water contamination was particularly pronounced in the Mount Wilson solar Doppler measurements.
Many scientific telescopes have spectrograph
s, which measure photons as a function of wavelength
or frequency
, with typical resolution on the order of a nanometer. Spectroscopic observations can be used in a myriad of contexts, including measuring the chemical composition and physical properties of astronomical objects as well as measuring object velocities from the Doppler shift of spectral lines. Unless they are corrected for, telluric contamination can produce errors or reduce precision in such data.
Telluric contamination can also be important for photometric
measurements.
. This is done by preparing a telluric correction function
, made by dividing a model spectrum
of a star by an observation of an astronomical photometric standard stars
. This function can then be multiplied by an astronomical observation at each wavelength
point.
While this method can restore the original shape of the spectrum
, the regions affected can be prone to high levels of noise
due to the low number of counts in that area of the spectrum.
Systematic error
Systematic errors are biases in measurement which lead to the situation where the mean of many separate measurements differs significantly from the actual value of the measured attribute. All measurements are prone to systematic errors, often of several different types...
of the astronomical spectra
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
by the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
.
Interference with astronomical observations
Most astronomical observations are conducted by measuring photonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s (electromagnetic waves) which originate beyond the sky. The molecules in the Earth's atmosphere, however, absorb
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
and emit their own light, especially in the visible and near-IR
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
portion of the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
, and any ground-based observation is subject to contamination from these telluric (earth-originating) sources. Water vapor, oxygen, and OH are some of the more important molecules in telluric contamination. Water contamination was particularly pronounced in the Mount Wilson solar Doppler measurements.
Many scientific telescopes have spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
s, which measure photons as a function of wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
or frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, with typical resolution on the order of a nanometer. Spectroscopic observations can be used in a myriad of contexts, including measuring the chemical composition and physical properties of astronomical objects as well as measuring object velocities from the Doppler shift of spectral lines. Unless they are corrected for, telluric contamination can produce errors or reduce precision in such data.
Telluric contamination can also be important for photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
measurements.
Telluric correction
It is possible to correct for the effects of telluric contamination in an astronomical spectrumSpectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
. This is done by preparing a telluric correction function
Function (mathematics)
In mathematics, a function associates one quantity, the argument of the function, also known as the input, with another quantity, the value of the function, also known as the output. A function assigns exactly one output to each input. The argument and the value may be real numbers, but they can...
, made by dividing a model spectrum
Model spectrum
A model spectrum is an artificial spectrum of a star made by recreating the absorption or emission features seen in the spectrum. A model spectrum will usually be of a specific spectral type of star and may include other properties of the star, such as a surrounding nebula, presence or lack of an...
of a star by an observation of an astronomical photometric standard stars
Photometric standard stars
Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system, measured very carefully...
. This function can then be multiplied by an astronomical observation at each wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
point.
While this method can restore the original shape of the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
, the regions affected can be prone to high levels of noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
due to the low number of counts in that area of the spectrum.
See also
- PollutionPollutionPollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
and light pollutionLight pollutionLight pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:... - InterferometryInterferometryInterferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
and astronomyAstronomyAstronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth... - SpectroscopySpectroscopySpectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
and spectrographSpectrographA spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
Further reading
- Christopher S. Carter, Herschel B. Snodgrass, and Claia Bryja, "Telluric water vapor contamination of the Mount Wilson solar Doppler measurements". September 30, 1991.