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An optical design for a wide-field optical spectrograph for TMT

We describe a preliminary optical design for a multi-object, wide-field, optical echellette spectrograph that is intended to serve a broad range of science. It will produce low-resolution, single-order spectra for survey-mode programs targeting as many objects as possible and also moderate-resolution, multiple-order spectra for a reduced number of targets. The design uses all refracting optics. The first optical element of the spectrograph is a wide-field corrector for the telescope that causes the chief rays to be perpendicular to the focal plane. The collimator, which has been designed on-axis, can then be duplicated to target multiple, off-axis fields in a multiple-barrel configuration. The collimator optics include an achromatic field lens group that forms a sharp pupil over the full optical band-pass (320-1000 nm), followed by a dichroic which splits the beam into a red and a blue channel. All remaining optical elements of the collimator, the gratings, the cameras, and the detectors are then optimized for red or blue wavelengths. Both red and blue channels of each beam of the spectrograph use reflection gratings to produce either a single-order spectrum at resolutions around R=λ/Δλ=1000 or a five-order, R>5000 echellette spectrum with prism cross-dispersion. Both modes can target objects anywhere in the collimated field of view. A direct imaging mode will also be provided.