  The InfraRed Imaging Spectrograph (IRIS) will be the first light adaptive
optics instrument on the Thirty Meter Telescope (TMT). IRIS is being built by a
collaboration between Caltech, the University of California, NAOJ and NRC
Herzberg. We present novel aspects of the Support Structure, Rotator and
On-Instrument Wavefront Sensor systems being developed at NRC Herzberg. IRIS is
suspended from the bottom port of the Narrow Field Infrared Adaptive Optics
System (NFIRAOS), and provides its own image de-rotation to compensate for
sidereal rotation of the focal plane. This arrangement is a challenge because
NFIRAOS is designed to host two other science instruments, which imposes strict
mass requirements on IRIS. We have been tasked with keeping the instrument mass
under seven tonnes which has resulted in a mass reduction of 30 percent for the
support structure and rotator compared to the most recent IRIS designs. To
accomplish this goal, while still being able to withstand earthquakes, we
developed a new design with composite materials. As IRIS is a client instrument
of NFIRAOS, it benefits from NFIRAOS's superior AO correction. IRIS assists
this correction by sensing low-order aberrations with an On-Instrument
Wavefront Sensor (OIWFS). The OIWFS consists of three independently positioned
natural guide star wavefront sensors that patrol a 2-arcminute field of view.
We expect tip-tilt measurements from faint stars within the IRIS imager focal
plane will further stabilize the delivered image quality. We describe how the
use of On-Detector Guide Windows (ODGWs) in the IRIS imager can be incorporated
into the AO correction. Finally, we present our strategies for acquiring and
tracking sources with this complex AO system, and for mitigating and measuring
the various potential sources of image blur and misalignment due to properties
of the mechanical structure and interfaces. (Abridged)
