Integral Field Spectroscopy

Integral field spectroscopy (sometimes called 3D spectroscopy) is a technique for obtaining moderate or high dispersion spectroscopy across a two-dimensional field of view (FoV). Integral field units (IFUs) may be configured as a front-end input to a conventional long slit spectrograph, or may be part of a dedicated integral field spectrograph system. Integral field spectrographs are characterised by their total FoV, spatial sampling (sometimes called spaxel size) and spectral range & resolution. There are at least three ways of reformatting a two-dimensional field of view using an IFU which are illustrated in the graphic below, all of which produce a 3D data cube with two spatial and one spectral dimensions.

CfAI has experience in designing and building all three types of IFU, with particular emphasis on the Lenslet+fibre and Image slicer approaches. Examples of the former are the TEIFU instrument for the 4.2m William Herschel Telescope [1], the GMOS integral field units for Gemini-N and Gemini-S [2], and the IMACS IFU for the Magellan telescope [3]. Examples of the latter technique are the GNIRS IFUs for Gemini-S [4] and Gemin-N [5], the NIRSpec IFU on JWST [6] and the 24 IFUs build for the world’s first near-infrared multi-object IFU system KMOS on the ESO VLT [7]. Pictures of some of the construction phases of these instruments are shown below. Current technology developments in this area are focussed on developing cooled infrared fibre cables for Lenslet+fibre IFUs, and improving the surface roughness and spatial resolution of image slicers.

[1] TEIFU: a high-resolution integral field unit for the William Herschel Telescope
[2] Integral Field Spectroscopy with the Gemini Multiobject Spectrograph. I. Design, Construction, and Testing
[3] Design and construction of the IMACS-IFU: a 2000-element integral field unit
[4] New techniques for integral field spectroscopy – I. Design, construction and testing of the GNIRS IFU
[5] The HR image slicer for GNIRS at Gemini North: optical design and performance
[6] Prototyping of diamond machined optics for the KMOS and JWST NIRSpec integral field units
[7] The KMOS Integral Field System: fabrication, alignment, and test of 1000+ optical surfaces