Integral Field Spectroscopy (IFS) is a technique to produce a spectra over a contiguous two-dimensional field, producing as a final data product a three-dimensional data cube of the two spatial co-ordinate axes plus an additional spectral axis, usually in wavelength. Although existing techniques, such as stepping a longslit spectrograph or scanning a Fabry-Perot device, can produce such a data cube, the IFS technique collects the data simultaneously with obvious savings in observing efficiency. However, IFS has only recently approached maturity as a hardware technique.
The technique started with the use of lenslet arrays without fibres, but the lack of a reformatting ability resulted in a short spectral range. The use of fibres improves on the lenslet-only technique, since the field can be reformatted into a pseudo-slit which can be dispersed by conventional spectrographs, and allows an IFS capability to be retrofitted to existing spectrographs. The earliest versions used bare fibres (e.g. INTEGRAL on WHT) but this suffers from inefficient coupling to the telescope and incomplete field coverage due to gaps between the fibre cores. Both these problems can be solved by coupling the fibres to micro-lenses. Despite its greater technical difficulty, this technique has been successfully prototyped, e.g. SMIRFS on UKIRT and TEIFU on the WHT. For infrared instruments working in cryogenic or space environments which are hostile to fibres, the technique of image slicing has been developed. Here the field is sliced into one-dimensional sections which are then reformatted into a near-continuous long slit.