Integral field spectrograph
Encyclopedia
An integral field spectrograph or a spectrograph equipped with an integral field unit (IFU) is an optical instrument combining spectrographic and imaging capabilities, used to obtain spatially resolved spectra in astronomy
and other fields of research such as bio-medical science and earth observation
(or remote sensing
). For a brief summary, see, and for a wider perspective.
Integral field spectroscopy (IFS) has become an important sub-discipline of astronomy with the proliferation of large aperture, high-resolution telescopes where there is a need to study the spectra of extended objects as a function of position, or of clusters of many discrete stars or point sources in a small field. Such investigations have previously been carried out with long-slit spectrographs
in which the spectrum is dispersed perpendicular to the slit, and spatial resolution is obtained in the dimension along the slit. Then by stepping the position of the slit, the spectrum of points in the imaged field can be obtained, but the process is comparatively slow, and wasteful of potentially restricted telescope time. Integral field spectrographs are used to speed up such observations by simultaneously obtaining spectra in a two-dimensional field. As the spatial resolution of telescopes in space (and also of ground-based instruments using adaptive optics) has rapidly improved in recent years, the need for such multiplexed instruments has become more and more pressing.
A recent development is diverse field spectroscopy which combines the benefit of IFS with multi-object spectroscopy (MOS). MOS is used to collect light from many discrete objects over a wide field. This does not record spatial information - just the spectrum of the total light collected within each sampling aperture (usually the core of a positionable optical fibre or a slitlet cut in a mask at the telescope focus). in contrast, IFS obtains complete, spatially-resolved coverage over a small field. The MOS targets are generally faint objects at the limits of detection such as primaeval galaxies. As telescopes get bigger it is apparent that these actually have blobby and confused structure that requires the observer to carefully select which parts of the field will be passed through to the spectrographs since it is not feasible to carpet the whole field with a single huge IFU. DFS is an instrument paradigm that allows the observer to select arbitrary combinations of contiguous and isolated regions of the sky to maximise observing efficiency and scientific return. Various technologies are under development including robotic switch-yards and photonic optical switches.
Other techniques can achieve the same ends at different wavelengths. The ACIS Advanced CCD Imaging Spectrometer
on NASA's Chandra X-Ray Observatory
is an example that obtains spectral information by direct measurement of the energy of each photon. This approach is much harder at longer wavelengths because the photons are less energetic. However progress has been made even at optical and near-infrared wavelengths using pixellated detectors such as superconducting tunnel junctions. At radio wavelengths, simultaneous spectral information is obtainable with heterodyne receivers.
More generally, integral field spectroscopy is a subset of 3D-imaging techniques (also known as hyperspectral imaging
and - erroneously - 3D spectroscopy). Other techniques rely on generation of a path difference between interfering beams using electro-mechanical scanning techniques. Examples include Fourier transform spectroscopy
employing a Michelson interferometer
layout and Fabry–Pérot interferometry. Although, to a first order of approximation, all such techniques are equivalent in that they generate the same number of resolution elements in a datacube (with axes labelled by the two-spatial coordinates plus wavelength) in the same time, they are not equivalent when sources of noise are considered. For example, scanning instruments, although requiring fewer costly detector elements, are inefficient when the background is varying because, unlike IFS, the exposure of the signal and background are not made at the same time. For bio-medical science, in vivo studies also require simultaneous data collection.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
and other fields of research such as bio-medical science and earth observation
Earth observation
Earth observation is the gathering of information about planet Earth’s physical, chemical and biological systems. It is used to monitor and assess the status of, and changes in, the natural environment and the built environment. In recent years, Earth observation has become technologically more and...
(or remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
). For a brief summary, see, and for a wider perspective.
Integral field spectroscopy (IFS) has become an important sub-discipline of astronomy with the proliferation of large aperture, high-resolution telescopes where there is a need to study the spectra of extended objects as a function of position, or of clusters of many discrete stars or point sources in a small field. Such investigations have previously been carried out with long-slit spectrographs
Long-slit spectroscopy
In astronomy, Long-slit spectroscopy involves observing an elongated celestial object through an elongated slit aperture, and refracting this light with a prism or diffraction grating...
in which the spectrum is dispersed perpendicular to the slit, and spatial resolution is obtained in the dimension along the slit. Then by stepping the position of the slit, the spectrum of points in the imaged field can be obtained, but the process is comparatively slow, and wasteful of potentially restricted telescope time. Integral field spectrographs are used to speed up such observations by simultaneously obtaining spectra in a two-dimensional field. As the spatial resolution of telescopes in space (and also of ground-based instruments using adaptive optics) has rapidly improved in recent years, the need for such multiplexed instruments has become more and more pressing.
A recent development is diverse field spectroscopy which combines the benefit of IFS with multi-object spectroscopy (MOS). MOS is used to collect light from many discrete objects over a wide field. This does not record spatial information - just the spectrum of the total light collected within each sampling aperture (usually the core of a positionable optical fibre or a slitlet cut in a mask at the telescope focus). in contrast, IFS obtains complete, spatially-resolved coverage over a small field. The MOS targets are generally faint objects at the limits of detection such as primaeval galaxies. As telescopes get bigger it is apparent that these actually have blobby and confused structure that requires the observer to carefully select which parts of the field will be passed through to the spectrographs since it is not feasible to carpet the whole field with a single huge IFU. DFS is an instrument paradigm that allows the observer to select arbitrary combinations of contiguous and isolated regions of the sky to maximise observing efficiency and scientific return. Various technologies are under development including robotic switch-yards and photonic optical switches.
Other techniques can achieve the same ends at different wavelengths. The ACIS Advanced CCD Imaging Spectrometer
Advanced CCD Imaging Spectrometer
ACIS, the AXAF CCD Imaging Spectrometer, is an instrument built by a team from the Massachusetts Institute of Technology's Center for Space Research and the Pennsylvania State University for the Chandra X-ray Observatory...
on NASA's Chandra X-Ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
is an example that obtains spectral information by direct measurement of the energy of each photon. This approach is much harder at longer wavelengths because the photons are less energetic. However progress has been made even at optical and near-infrared wavelengths using pixellated detectors such as superconducting tunnel junctions. At radio wavelengths, simultaneous spectral information is obtainable with heterodyne receivers.
More generally, integral field spectroscopy is a subset of 3D-imaging techniques (also known as hyperspectral imaging
Hyperspectral imaging
Hyperspectral imaging collects and processes information from across the electromagnetic spectrum. Much as the human eye sees visible light in three bands , spectral imaging divides the spectrum into many more bands...
and - erroneously - 3D spectroscopy). Other techniques rely on generation of a path difference between interfering beams using electro-mechanical scanning techniques. Examples include Fourier transform spectroscopy
Fourier transform spectroscopy
Fourier transform spectroscopy is a measurement technique whereby spectra are collected based on measurements of the coherence of a radiative source, using time-domain or space-domain measurements of the electromagnetic radiation or other type of radiation....
employing a Michelson interferometer
Michelson interferometer
The Michelson interferometer is the most common configuration for optical interferometry and was invented by Albert Abraham Michelson. An interference pattern is produced by splitting a beam of light into two paths, bouncing the beams back and recombining them...
layout and Fabry–Pérot interferometry. Although, to a first order of approximation, all such techniques are equivalent in that they generate the same number of resolution elements in a datacube (with axes labelled by the two-spatial coordinates plus wavelength) in the same time, they are not equivalent when sources of noise are considered. For example, scanning instruments, although requiring fewer costly detector elements, are inefficient when the background is varying because, unlike IFS, the exposure of the signal and background are not made at the same time. For bio-medical science, in vivo studies also require simultaneous data collection.
External links
- For the Caltech Palomar 5m Telescope. The Palomar Integral Field Spectrograph
- arXiv:astro-ph/0606597v2 "The Integral Field Spectrograph of SPHERE: the Planet Finder for VLT"
- PACS paper The Photodetector Array Camera and Spectrometer (PACS) for the Herschel Space ObservatoryHerschel Space ObservatoryThe Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....
- The Integral Field Spectroscopy wiki
- NIRSPEC spectrometer for JWST