Exchanging the NOTCam entrance window in March 2008 plus extensive baking of the whole cryostat, resulted in a better than ever vacuum. When cooling down with LN2 there were problems, which are now fully understood to be related to thermal stresses around fill-tube o-rings. The cryostat recovered these losses well. A holding time test showed better than ever results. The average pressure settled from 1.1E-4 mbar in April to 5.0E-5 mbar in September from when on it has stayed practically constant. Thus, NOTCam stayed nearly 1 year cold without any problems, its longest period ever.
The previous plan of exchanging O-rings has therefore become less of an issue, and in principle the cryostat vessel now behaves very well. What remains to be improved is the procedure for cooling down from a warm state to avoid thermal stresses around the O-rings in the filling tubes. If we can avoid vacuum losses during cool-down we probably could keep the instrument cold continuously for significantly longer than 1 year.