Integral field spectroscopy with the Gemini Multiobject Spectrographs

We describe the integral field unit (IFU) which converts the Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope to an integral field spectrograph, which produces spectra over a contiguous field of view of 7x5 arcsec with spatial sampling of 0.2 arcsec over the wavelength range 0.4x1.0 mum. GMOS is converted to this mode by the remote insertion of the IFU into the beam in place of the masks used for the multiobject mode. A separate field of half the area of the main field, but otherwise identical, is also provided to improve background subtraction. The IFU contains 1500 lenslet-coupled fibres and was the first facility of any type for integral field spectroscopy employed on an 8/10 m telescope. We describe the design, construction and testing of the GMOS IFU and present measurements of the throughput both in the laboratory and at the telescope. We compare these with a theoretical prediction made before construction started. All are in good agreement with each other, with the on-telescope throughput exceeding 60% (averaged over wavelength). Finally we show an example of data obtained during commissioning to illustrate the power of the device.