Integral field spectrograph
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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).
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 spectroscopic 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.
In this approach, an image is sliced (using for example a Bowen image slicer) in the image-plane and re-arranged such that different parts of the image all fall onto a slit and a dispersing element, such that a spectrum is obtained for a larger area of interest. Another way to think of this is that the slit is optically cut into smaller pieces and re-imaged onto the image-plane at multiple locations.
In this type of IFU, a lenslet array is placed in the image plane, essentially acting as pixels. All beams generated by this lenslet array are then fed through a dispersive element and imaged by a camera, resulting in a spectrum for each individual lenslet.
Here, the light of targets of interest is captured by an array of fibers. The other end of the fibers are arranged along a slit such that one obtains a spectrum for each fiber.
This technique is used on INTEGRAL at the William Herschel Telescope, and is also the method for many upcoming galaxy surveys including SAMI at the Australian Astronomical Observatory and MaNGA which is one of the surveys making up the next phase of the Sloan Digital Sky Survey.
Diverse Field Spectroscopy
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 primeval 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 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.
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