The Infrared Imaging Spectrograph (IRIS) for TMT: achieving high sky coverage through the On-Instrument Wavefront Sensor design

IRIS is a diffraction-limited imaging spectrograph designed for first light of the Thirty Meter Telescope (TMT). It is fed Multi-Conjugate Adaptive Optics (MCAO)-corrected light from the Narrow-Field Infrared Adaptive Optics System (NFIRAOS). IRIS, however, is responsible for sensing the low order modes: tip, tilt, focus, and plate scale modes to which the NFIRAOS Laser Guide Star (LGS) Wavefront Sensors (WFSs) are blind. During the IRIS final design, we developed a noise-equivalent angle budget that included the optical throughput of the telescope, NFIRAOS and OIWFS, the detector pixel size, quantum efficiency, readnoise and excess shot noise, diffraction including optical aberrations, and sky and thermal backgrounds. The noise equivalent angle was calculated for the near-infrared star brightness corresponding to 50% sky coverage at the North Galactic Pole for an exposure time required to meet the overall TMT wavefront error budget. We present the trade studies, performance budgets and simulation results which drove the final design of the optics, mechanics and detectors of the IRIS OIWFS system.