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An integral field unit (IFU) is an instrument that allows a "normal" spectrograph, i.e., with a slit opening, to capture data across a two-dimensional field. The combined instrument is called an integral field spectrograph.
A typical spectrograph collects data from a line through an image (one dimension) and offers spectral data over one spatial dimension. The integral field unit rearranges points of light so that the slit includes data across a two-dimensional area, but at a relatively low spatial resolution. Spectrographs are often built so they can be used with or without the integral field unit.
Methods used:
An image slicer directs light from different parts of the image into a slit such that parts of the image across two dimensions are all fed through the one dimension of the slit width. UVES of the ESO VLT does this.
A lenslet array has a lens for each pixel. SAURON of the William Herschel Telescope (WHT) uses this.
Fibers uses optical fibers to direct the image into a single slit. INTEGRAL, another IFU for WHT, uses this.
The phrase IFU observation naturally refers to a spectrographic observation using an IFU.