The wide field optical spectrograph (WFOS) for TMT: fiber-WFOS optical design (Conference Presentation)

TMT’s wide field optical spectrograph is a multi-object, first-light instrument with broad continuous wavelength coverage (0.310 – 1.0 m) at a moderate spectral resolution of R = 5000. The international WFOS design team has recently completed the downselect of two design approaches: a slicer-based monolithic architecture and a fiber-based modular concept. We present here the end-to-end conceptual design for the fiber-based optical spectrograph. Included are the front-end focal reduction optics for coupling light into the fibers, the spectrograph collimator and camera optics, and the dispersive architecture for each color channel. The highly multiplexed fiber-WFOS presents a unique design challenge in keeping costs for the modular spectrographs low while maintaining performance gains afforded by the TMT, and in particular the TMT plus ground-layer adaptive optics (GLAO). A full performance analysis including predicted spectral resolution and throughput is presented for the design.