  The Magellan InfraRed Multi-Object Spectrograph (MIRMOS) is a planned next
generation multi-object and integral field spectrograph for the 6.5m Magellan
telescopes at Las Campanas Observatory in Chile. MIRMOS will perform
R$\sim$3700 spectroscopy over a simultaneous wavelength range of 0.886 -
2.404$\mu$m (Y,J,H,K bands) in addition to imaging over the range of 0.7 -
0.886$\mu$m. The integral field mode of operation for MIRMOS will be achieved
via an image slicer style integral field unit (IFU) located on a linear stage
to facilitate movement into the beam during use or storage while operating in
multi-object mode. The IFU will provide a $\rm \sim20"\times26"$ field of view
(FoV) made up of $\rm0.84"\times26"$ slices. This will be the largest FoV IFS
operating at these wavelengths from either the ground or space, making MIRMOS
an ideal instrument for a wide range of science cases including studying the
high redshift circumgalactic medium and emission line tracers from ionized and
molecular gas in nearby galaxies. In order to achieve the desired image quality
and FoV while matching the focal ratio to the multi-object mode, our slicer
design makes use of novel freeform surfaces for the pupil mirrors, which
require the use of high precision multi-axis diamond milling to manufacture. We
present here the optical design and predicted performance of the MIRMOS IFU
along with a conceptual design for the opto-mechanical system.
