Instrument

During observations problems arose with positioning filter wheel 2. This filter wheel contains the main broad-band filters but spare JHK filter had been put in the stop wheel in the past just for these kind of problems. The rest of the run was performed with filter wheel 2 fixed in the ``open'' position, while only filter wheel 1 was used together with the spare JHK filters in the stop wheel. The instrument setup scripts were temporarily modified to avoid accidental movement of wheel 2.

No definite diagnostics could be made so it was decided to warm up and open the instrument. It was found that the flexible coupling between the motor and the worm gear for filter wheel 2 had broken. The reason for this remains unclear. Filter wheel 2 is the wheel that is moved the most, but both wheel and worm gear are easy to move, so it was suggested that perhaps thermal stresses played a role since the motor support is aluminium and the worm gear is steel. New flexible couplings were ordered and subsequently installed for all wheels as a preventive measure.

As the bearings of the different wheels were exchanged 3 and a half years ago it was decided to take the opportunity to replace them. All the bearings looked as new, and it is thought that we do not need to exchange them for preventive reasons as often as believed earlier. The earlier estimate was based on the first years of experience with the instrument, during a phase where the wheels were moved much more often than currently.