The waveplate blanks have just arrived from United Lens. These take the place of the waveplates in the beam when one or both of the waveplates are out. The thin plate (12 mm) is used when only the half-wave plate is used (i.e. linear polarimetry) and the thick plate (24 mm) used in non-polarimetric modes. These ensure that the optical path length through the collimator remains constant in all modes.
07 October 2003:
The polarizing waveplates have arrived from Germany. Below are the quarter-wave and the half-wave plates.