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GNIRS On-Instrument Wavefront Sensor (OIWFS)
The OIWFS is an optical sub-system built by the Institute for Astronomy (University of Hawaii). The light path is also shown in the optical schematic. Light from the guide field - anything in a 3 arcminute diameter field that misses the pick-off mirror - enters the OIWFS field lens and then passes to a collimator doublet and then a 2-axis gimbal mirror.
The gimbal mirror can be tilted precisely to direct light from any part of the guide field to the rest of the OIWFS optics. The usable field is roughly 10 arcsecs diameter, which is sufficient to acquire individual guide stars. Light from the selected patch of sky is reflected off the gimbal mirror, through a second doublet, and the guide star is re-imaged on the filter wheel, which contains JHK bandpass filters as well as an ultra-wide filter covering the entire 1.0-2.4 microns interval.
From the filter wheel, the light is focussed on the detector array; a single image of the guide star is formed there. Only a small portion of the array is read out, allowing rapid data rates, which may permit the OIWFS to provide high-speed tip-tilt correction in addition to slower flexure and tracking corrections.
The OIWFS has not yet been commissioned.