The Infra-Red Imaging Spectrograph (IRIS) is one of the three first light
instruments for the Thirty Meter Telescope (TMT) and is the only one to
directly sample the diffraction limit. The instrument consists of a parallel
imager and off-axis Integral Field Spectrograph (IFS) for optimum use of the
near infrared (0.84um-2.4um) Adaptive Optics corrected focal surface. We
present an overview of the IRIS spectrograph that is designed to probe a range
of scientific targets from the dynamics and morphology of high-z galaxies to
studying the atmospheres and surfaces of solar system objects, the latter
requiring a narrow field and high Strehl performance. The IRIS spectrograph is
a hybrid system consisting of two state of the art IFS technologies providing
four plate scales (4mas, 9mas, 25mas, 50mas spaxel sizes). We present the
design of the unique hybrid system that combines the power of a lenslet
spectrograph and image slicer spectrograph in a configuration where major
hardware is shared. The result is a powerful yet economical solution to what
would otherwise require two separate 30m-class instruments.Comment: 15 pages, 11 figure