The major change to take place is a second filter wheel has been installed. This wheel is between the field lens and the pupil slide, located close to the pupil. It has 8 positions like the first wheel; however, one position holds a polarizer which is slightly larger than the standard filter so this position can only hold the polarizer. The polarizer therefore can be used in conjuction with any filter in wheel #1. However, to make polarimetric observations would also require additional optics such as a rotating half-wave plate which is currently not available at the 88-inch.
Since both wheels must have an ``OPEN'' position to allow for using only one filter at a time, and the polarizer occupies one position in wheel 2, this leaves 13 positions for filters. Wheel 1 will hold the standard broadband filters J, H, K, K', H+K, and probably the narrowband 2.12 micron molecular hydrogen filter and the 2.26 micron continuum filter. This leaves six positions in the second wheel available for other narrowband or special purpose filters. If you require special filters for your observing run, you must clearly state this in your proposal, in the ``Additional Information'' section. Usually only one filter change at the most is scheduled per semester, so you must have your filters ready well before your run and deliver them to the summit so that they can be installed.
The original filter wheel has had problems during the past couple months. During its last CFHT run the wheel apparently stuck in one position. The dewar was opened and the wheel and motor adjusted. Since then, the wheel has mostly worked, but there have been intermittent problems. The symptom of the filter motion failure is that the detector is vignetted by the edge of the filter holder, and one sees the curved edge cutting off the flux from part of the detector. To recover from this, one must re-home the wheel and redo the motion. There is some indication that the problem occurs between the K and K' filters (positions 3 and 4). To help avoid problems, one could try to avoid moving between these filters. The filter wheel tries to move the minimum distance between filters, so if one wants to move from #3 to #4, first command to move to position 7, then to position 4.
To move the filters, one uses the same ``filter'' command as before. The program automatically positions both wheels. When selecting filters in wheel 1 (positions 1-7), wheel 2 is moved to the OPEN position, and similarly for wheel 2. If one wants the polarizer along with a filter in wheel 1, for example for the filter in position 1 (J), one would use the command ``fi 1 pol'' to set the wheels. To remove the polarizer, one would type ``fi 1''. To home both wheels, the command ``fi home'' is used as before. One can also home the individual wheels with the commands ``fi home1'' or ``fi home2''.
Last modified 1996 August 17