The IMB acquisition system contains the Tip-Tilt sensor mounted on an
X-Y stage at the bottom of the IMB. This system views the telescope
field in optical light through the dichroic reflexer, and allows
offset guiding and selection of bright tip-tilt reference stars. The
optical transmission function of the dichroic mirror is plotted in
Figure 59 in Appendix L. The travel of
the X-Y stage is 
mm in each direction in the f/18
(5''/mm) telescope beam. A focussing unit mounted on the X-Y stage
directly in front of the Tip-Tilt sensor changes the image scale,
provides a pupil mask that acts as an optical sky baffle, and allows for
focussing of the optical image independent of the infrared image.