Integral field spectrographs (IFS) combine spectrographic and imaging capabilities in the optical or infrared wavelength domains (0.32 μm – 24 μm) to get from a single exposure spatially resolved spectra in a bi-dimensional region. The name originates from the fact that the mesurements result from integrating the light on multiple sub-regions of the field. Developed at first for the study of astronomical objects, this technique is now also used in many other fields, such bio-medical science and Earth remote sensing. Integral field spectrography is part of the broader category of snapshot hyperspectral imaging techniques, itself a part of hyperspectral imaging.
Rationale
With the notable exception of individual stars, most astronomical objects are spatially resolved by large telescopes. For spectroscopic studies, the optimum would then be to get a spectrum for each spatial pixel in the instrument field of view, getting full information on each target. This is loosely called a datacube from its two spatial and one spectral dimensions. Since both visible charge-coupled devices (CCD) and infrared detector arrays (staring arrays) used for astronomical instruments are bi-dimensional only, it is a non-trivial feat to develop spectrographic systems able to deliver 3D data cubes from the output of 2D detectors. Such instruments are usually christened 3D spectrographs in the astronomical field and hyperspectral imagers in the non-astronomical ones.
Hyperspectral imager can be broadly classified in two groups, scanning and non-scanning. The first contains the instruments that build the datacube by combining multiple exposures, scanning along a space axis, a wavelength axis or diagonally through it. Examples include push broom scanning systems, scanning Fabry-Perot and Fourier transform spectrometers. The second group includes the techniques that acquire the whole datacube in a single shot, snapshot imaging spectrometers. Integral field spectrography (IFS) techniques were the first snapshot hyperspectral imaging techniques to be developed. Since then, other snapshot hyperspectral imaging techniques, based for example on tomographic reconstruction or compressed sensing using a coded aperture, have been developed.
One major advantage of the snapshot approach for ground-based telescopic observations is that it automatically provides homogenous data sets despite the unavoidable variability of Earth’s atmospheric transmission, spectral emission and image blurring during exposures. This is not the case for scanned systems for which the data cubes are built by a set of successive exposures. IFS, whether ground or space based, have also the huge advantage to detect much fainter objects in a given exposure than scanning systems, if at the cost of a much smaller sky field area.
After a slow start from the late 1980s on, Integral field spectroscopy has become a mainstream astrophysical tool in the optical to mid-infrared regions, addressing a whole gamut of astronomical sources, essentially any smallish individual object from Solar System asteroids to vastly distant galaxies.
Methods
Integral field spectrographs use so-called Integral Field Units (IFUs) to reformat the small square field of view into a more suitable shape, which is then spectrally dispersed by a grating spectrograph and recorded by a detector array. There are currently three different IFU flavors, using respectively a lenslet array, a fiber array or a mirror array.
Lenslet array
An enlarged sky image feeds a mini-lens array, typically a few thousands identical lenses each about 1 mm in diameter. The lenslet array output is a regular grid of as many small telescope mirror images, which serves as the input for a multi-slit spectrograph that delivers the data cubes. This approach was advocated in the early 1980s, with the first lenslet-based optical TIGER IFS observations in 1987.
Pros are 100% on-sky spatial filling when using a square or hexagonal lenslet shape, high throughput, accurate photometry and an easy to build IFU. A significant con is the suboptimal use of precious detector pixels (~ 50% loss at least) in order to avoid contamination between adjacent spectra.
Instruments like the Spectrographic Areal Unit for Research on Optical Nebulae (SAURON) on the William Herschel Telescope and the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) IFS subsystem on European Southern Observatory (ESO)'s Very Large Telescope (VLT) use this technique.