This system serves as a backup in case of failure of the tip-tilt system or in the case that no suitable guide stars are found in the range of the tip-tilt stage. It provides no tip-tilt capability. The field of the camera is very limited. It cannot be used close to the axis because of vignetting of the IRS beam. On the other hand, precision offsetting (follow on) is available with this probe. And the coverage is generous: the probe scans a somewhat irregularly shaped rectangular field to the north of the optical axis about 12X15 arcmins.