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 filters and apertures.

From the filter wheel, light enters the Shack-Hartmann optics, which are mounted on a "snout" on the detector mount. The Shack-Hartmann optics form a pupil image at a shallow four-facet prism, then re-image the star on the detector. Because the light has passed through the Shack-Hartmann prism, four images of the star are actually formed, corresponding to 1/4 of the pupil each. Only a small portion of the array is read out, allowing rapid data rates, which permit the OIWFS to provide high-speed tip-tilt and focus correction in addition to slower flexure and tracking correction. For fainter guide stars, fast correction is limited to tip-tilt and does not include focus.

The OIWFS filter wheel contains standard JHK filters. In principle, users should select the filter that provides the best signal to noise on the guide star (normally H), since the telescope's acquisition and guide (A&G) system adjusts the OIWFS gimbal mirror position to compensate for differential refraction between the guide wavelength and the observing wavelength.

Last update December 22, 2003; Bernadette Rodgers and Sybil Adams