Canceled talk replaced by: Configurable slit unit versus slit mask exchanger trade study for the TMT wide field optical spectrometer, Alastair Heptonstall, Thirty Meter Telescope

We present a trade study on design changes to the Wide Field Optical Spectrometer for the Thirty Meter Telescope. WFOS is planned as a first light instrument providing imaging and multi-slit spectroscopy over the wavelength range 0.31 to 1µm across a field of view of 8.3 by 3 arcminutes. The baseline prior to the trade study used laser cut metal slit masks at the focal plane to enable observation of ~50 to 80 objects simultaneously. The configurable slit unit design uses multiple knife edges mounted on computer-controlled bars to create up to 88 reconfigurable slits, enabling the ability to adapt to seeing conditions or to respond to targets of opportunity. We detail here the decision criteria, and science case analysis used by the WFOS team to decide to change the baseline design of WFOS to incorporate a CSU.