An integral field spectrograph, or a spectrograph equipped with an integral field unit, 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.
Rationale
Integral field spectroscopy 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 has rapidly improved in recent years, the need for such multiplexed instruments has become more and more pressing.
Methods
Image slicer
In this approach, an image is sliced 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. An instrument using this technique is for example UVES at the Very Large Telescope.
Lenslet array
In this type of IFU, a lenslet array is placed in the spectrograph entrance slits plane, essentially acting as spatial pixels or spaxels. All beams generated by the lenslet array are then fed through a dispersive element and imaged by a camera, resulting in a spectrum for each individual lenslet. Instruments like SAURON on the William Herschel Telescope and the SPHERE IFS subsystem on the VLT use this technique.
Fibers
Here, the light of targets of interest is captured by an array of fibers, forming the spectrographs entrance slits plane. The other end of the fibers are arranged along a single slit such that one obtains a spectrum for each fiber. This technique is used by instruments in many telescopes , and particularly in currently ongoing large surveys of galaxies, such as CALIFA at the Calar Alto Observatory, 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 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. 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 a 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 approaches
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.
Hyperspectral imaging
More generally, integral field spectroscopy is a subset of 3D-imaging techniques. 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 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.
