Integral field spectrograph

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.

An instrument using this technique is for example UVES at the Very Large Telescope

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.

Instruments like SAURON on the William Herschel Telescope and the SPHERE IFS subsystem on the VLT use this technique.

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