All infrared dewars mount on a box attached to the Cassegrain focus of the 2.3 m telescope that is known as the Instrument Mounting Box (IMB; see Fig. 23). Two dewars can mount on the IMB at the same time and a dichroic mirror in the IMB can be positioned to direct infrared light from the telescope to either dewar. The IMB output ports are identified by the labels A-D. From the Cassegrain Access Platform, with the control electronics racks on your left, the port positions facing you is `A', the one to your right (not used) is `B', the back position is `C', and the control electronics mount on face `D'. A dichroic refexer directs the infrared beam to the selected IMB port while an acquisition system views the same field in optical light. Auxiliary modules can be mounted above the dichroic reflexer in the IMB. A calibration lamp module is available, and a polarimetry module is under construction.
A manual dust cover is located in the mounting flange at the top of the IMB. The open and closed positions are clearly marked on the flange, and it should be used.