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Spectrometers and Spectrographs in Space and on Earth

Spectroscopy is the study of spectra, spectrography is the recording of the spectra, and spectrometry is the measure of spectra. The three terms are used almost interchangeably in astronomy, astrophysics and other disciplines.

Spectra (plural of spectrum) are ways to look at light (or electromagnetic waves) where different wavelengths are represented separately. An example we have all seen is a rainbow. Light from the Sun includes photons of all wavelengths that mix to make white light. When light goes through a prism or a water drop, it gets dispersed into its different colours, which correspond to different wavelengths.

The colors of the rainbow, in order from the longest to the shortest wavelengths, are as follows:

  • Violet: Approximately 380 to 420 nanometers (nm)
  • Indigo: Approximately 420 to 450 nm
  • Blue: Approximately 450 to 495 nm
  • Green: Approximately 495 to 570 nm
  • Yellow: Approximately 570 to 590 nm
  • Orange: Approximately 590 to 620 nm
  • Red: Approximately 620 to 750 nm

The rainbow is formed when light is refracted, or bent, as it passes through water droplets in the atmosphere, like after rainfall. This bending of light separates the different colors, creating the beautiful spectrum of colors we see in a rainbow.

Rainbow: the spectrum of white light. Credit: Dreamstime

Different chemical compounds shine or reflect predominantly certain wavelengths. Using spectroscopy, scientists can split the light coming from a given object and infer its composition.

Imaging spectrographs can take images where each pixels can be turned into a “rainbow”. This means you can make a map of a region and get the image at each map location.

The entire electromagnetic spectrum is too broad to be split by a single instrument. Therefore, specific spectrographs are developed for the ultraviolet, visible, infrared, etc.

Ultraviolet (UV) spectrometers have various potential Earth applications due to their ability to analyse ultraviolet radiation from different sources. Some possible applications include:

  • UV spectrometers can be used to study the Earth’s atmosphere or the atmospheres of other solar system objects. On Earth, the stratosphere and upper atmosphere interact with UV radiation creating interesting UV spectra. These spectrometers can help monitor ozone concentrations, study atmospheric composition, and investigate processes like ozone depletion and photochemical reactions.
  • A related application is environmental monitoring. UV spectrometers can be employed to measure UV radiation levels in the environment, which are crucial for monitoring environmental health and understanding the impact of UV radiation on ecosystems, plants, and aquatic life. They can aid in assessing UV radiation’s role in climate change and its effects on sensitive environments.
  • UV spectrometers can contribute to space weather monitoring by analysing solar UV radiation. Understanding the Sun’s UV emissions and their impact on planetary atmospheres and magnetospheres is essential for space weather prediction and protecting space probes, and satellite for communication and navigation systems.
  • On Earth UV spectrometers can be used in industrial processes, such as semiconductor manufacturing where UV curing processes can be monitored to optimize UV radiation exposure for precise and controlled results.
  • Health and safety applications is another area where UV spectrometers can be used. Examples include monitoring UV radiation exposure for workers in outdoor environments, assessing UV protection in sunscreen products, and studying the impact of UV radiation on human health.
  • In addition to space missions to other planets and moons, UV spectrometers can be employed in ground-based research in astronomy to study the UV spectra of stars, galaxies, and other celestial objects.

JUICE carries a UV spectrometer called UVS, which operates in the range of 55-210 nm. It has spectral resolution <0.6 nm, which means it can “slice” light in slices 0.6 nm in width. Approximately, it will produce (210–55)/0.6 = 258 slices.
Imagine a rainbow with 258 “colours”?!

External view of the UVS instrument. Credit: CNES

Spectrometers covering the visible and infrared (VIS+IR) wavelengths has various Earth applications due to their ability to analyse the electromagnetic spectrum across a wide range of wavelengths. The visible goes from 0.4 to 0.7 µm and the infrared continues to the range of 1 to a few µm. One µm is called a micron, which corresponds to 1000 nm. So, 0.4 µm is the same as 400 nm.

  • Like UV spectrometers, VIS+IR spectrometers can be used for environmental monitoring, to study and monitor air and water quality, detecting pollutants and greenhouse gases.
  • Agriculture applications include assessing crop health and monitoring vegetation, identifying stressors or nutrient deficiencies.
  • They can be used in geology and mining to analyse minerals and rocks, aiding in mineral exploration and geological mapping.
  • Remote sensing, carried out from satellites in orbit or space probes, can be employed for land and ocean observations, helping in disaster management, land-use planning, and coastal monitoring.
  • A VIS+IR spectrometer can also be used in climate studies of atmospheric composition and greenhouse gas concentrations, contributing to climate research and climate change assessment.
  • They can help in forestry in inventories and monitoring, assessing tree species, and measuring forest health.
  • VIS+IR spectrometers are used in food quality and safety assessment.
  • Biomedical applications include a variety of medical diagnostics, e.g. analysis of tissues and biomolecules.
  • Astronomers also use these types of spectrometers when studying celestial bodies and their spectral signatures.

On JUICE, the MAJIS spectrograph will cover the visible and infrared wavelengths from 0.4 to 5.7 µm, with spectral resolution of 3-7 nm. In this case we have approximately (5700–400)/5=1060 “colours” in each spectrum.

Optical Head (OH) of the MAJIS instrument, comprising the telescope, beam splitter, two gratings, return optics and two focal planes. The instrument aperture and the two radiators are protected here by red metal covers. Credit: IAS

Can you come up with uses for a spectrograph in your business solution?!

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