Integral field spectroscopy with the GEMINI multi-object spectrographs

The GEMINI Multiobject Spectrograph (GMOS), due for delivery in late 2000, will include a powerful integral field spectroscopic capability. The instrument scan switch to this mode by the remote insertion of an integral field unit (IFU) into the focal plane in place of multiobject masks. The initial implementation of the GMOS IFU will cover a field in excess of 50 square arcsec with a sampling of 0.2 arcsec via 1500 spatial elements with spectra covering up to 3000 pixels. The spectrum length may also be doubled by halving the field. A separate field is provided at fixed offset to facilitate accurate background subtraction. The system employs a fiber-lenslet technique that provides significant benefits over unlensed fiber reformatters and fiberless lenslet arrays. The specific advantages are unit filling factor, high throughput and long spectra. The IFU has been designed in the light of our experience with two other successful devices of this type. We summarize the design of the device and discuss how the IFU will be operated within the context of GMOS and the GEMINI telescopes. Finally, we present options for implementing IFUs with finer spatial resolution on GMOS.