A new design for integral field spectroscopy with 8-m telescopes

I propose a new design of integral field unit (IFU) for two-dimensional area spectroscopy. The unit acts as a coupler between the telescope and spectrograph to reformat a square or rectangular field into a long slit. Tn addition to providing good transmission, it avoids the geometrical losses inherent in previous designs based on fibre optics, lens array re-imaging and image slicing. The proposed design is a new type of image slicer in which the original two-dimensional image is sliced into narrow sub-images that are re-imaged side by side to form a long one-dimensional image at the spectrograph input. The new design is much more compact than previous designs making it easier to insert in the front of a spectrograph without any modification to the spectrograph support system. The design uses much smaller optics than previous designs. The small number of reflections - 4 or 6 depending on the telescope focal ratio - and the smaller instrument size, which simplify the cooling of the instrument, makes the design well suited for infrared spectroscopy. The application of this design to 8-m telescopes and its use with adaptive optics in the optical and infrared is discussed, particularly with respect to the multi-object spectrographs of the Gemini Telescopes project. In this case, the proposed design gives a field of 8.3''x11.4'' along with a background field of 2.6''x3.6'' with a spatial resolution of 0.03''x0.16'' and roughly 1200 spectral resolution elements in each spectrum. The spatial resolution is well suited to images produced by the GEMINI telescopes in tip/tilt mode (typically FWHM - 0.3'' in 10th percentile seeing) and with higher order adaptive optics. This design is also applicable to 4-m telescopes with - or without - adaptive optics. The possibility of inserting such an instrument in front of existing spectrographs is discussed.