integral field spectrograph

An integral field spectrograph (or integral field spectrometer) combines spectrography and imaging, producing a data across three dimensions (e.g., a data cube), two being spatial (the image) and one being the spectral data at each specified point. Its use is called integral field spectroscopy (IFS).

An integral field spectrograph is commonly a normal spectrograph plus an integral field unit (IFU). A normal spectrograph uses a slit to limit the data to that from a narrow, line-like portion of the focal plane. That line-like portion of the image is dispersed so that the spectrum is spread out along one dimension of the result. The other dimension shows such spectrums from points along the line created by the slit, i.e., the result covers one spatial dimension of the sky. A succession of such observations is needed to gather data covering both spatial dimensions of the sky. The IFU rearranges portions of the image fed into the spectrograph's slit so the narrow slit includes signal from a two-dimensional area of the sky. Doing so, it trades away some angular resolution.

IFU spectrographs are often designed so they can also operate without the IFU, offering normal spectrography as well. IFU methods used:

• Image slicer uses a series of adjacent narrow, rectangular reflectors that each span one dimension of the focal plane, each covering a slice of the image, their reflection such as to arrange the image so the slices of it are aligned end-to-end within the spectrograph's slit. I think a collimator at a focal plane is necessary. UVES of the ESO VLT does this.
• Lenslet array has a lens for each pixel. SAURON of the William Herschel Telescope (WHT) uses this.
• Fiber method uses optical fibers to direct the image into a single slit. INTEGRAL, used on the WHT uses this.