We present the conceptual design of the integral field unit (IFU) for Wide Field Optical Spectrograph (WFOS), one of the first-generation instruments on TMT. The IFU is a promising upgrade path of WFOS. The IFU has 4 image slicers with different slice widths of 1.5, 0.75, 0.5 and 0.25 arcsec. The slice length and the number of slices are 20 arcsec and 18 in all slicers. These slicers offer the field sizes of 27, 13.5, 9 and 4.5 × 20 arcsec2, respectively. This field variation covers sizes of galaxy, circum-galactic medium and inter-galactic medium. In the 0.25-arcsec width mode, the spectral resolution reaches R=13,635 without slit loss. Multilayer dielectric reflective coating with high reflectivity (> 98% at any wavelength) is made on all reflective surfaces, which offers high through put of the IFU (> 80%).
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