integral field unit

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. Among the IFU mechanisms used:

• An image slicer uses mirrors arranged to separately reflect slices of the image, placing the slices end-to-end along the length of the slit. UVES of the ESO VLT does this.
• A lenslet array has a 2-dimensional array of lenses, that direct light from the image to sections along the length of the slit. SAURON, formerly of the William Herschel Telescope (WHT) used this.
• Fibers uses optical fibers, arranged in a 2-dimensional array to pick up pixels of the image, outputting these pieces of the image along the length of the slit. INTEGRAL, another IFU formerly of the WHT, used this.

The phrases IFU observation and IFU spectroscopy naturally refer to a spectrographic observation using an IFU.