Maren Cosens, Nicholas P. Konidaris, Gwen C. Rudie, Andrew B. Newman, Leon Aslan, Robert Barkhouser, Christoph Birk, Julia Brady, Mathias Gumy, Tyson Hare, Stephen C. Hope, Charlie Hull, Karim Kaismoune, Daniel D. Kelson, Gerrad Killion, Alicia Lanz, Jocob McCloskey, Solange V. Ramirez, Cyril Rupf, William Schoenell, Patricio Schurter, Stephen A. Smee, Peter Spanoudakis, Jason E. Williams

The Magellan InfraRed Multi-Object Spectrograph (MIRMOS) will be a next generation multi-object (MOS) and integral field spectrograph (IFS) for the 6.5m Magellan telescopes at Las Campanas Observatory in Chile. MIRMOS will perform R~3700 spectroscopy over a simultaneous wavelength range of 0.886-2.404um (Y, J, H, and K bands) in addition to imaging over the range of 0.7-0.886um. Target selection in the MOS mode is achieved through a cryogenic mechanism capable of making up to 92 slits over a 13'x3' field of view (FoV). This mechanism can be reconfigured in real time to adjust slit widths for seeing conditions, change to a different mask, or form a long slit. The IFS mode of operation for MIRMOS will be achieved via an image slicer style integral field unit (IFU) which will provide an ~18"x26" FoV made up of 0.84"x26" slices. MIRMOS's design will allow it to address a wide range of science cases from performing spectroscopy of high-redshift galaxies as well as high signal-to-noise transmission spectroscopy of exoplanet atmospheres with the MOS/long slit mode, while also having the largest FoV IFS operating on a large telescope at these wavelengths. We will describe here the design of the MIRMOS instrument -- now at the end of the preliminary design phase.