The long lasting problems with loss of vacuum and short holding time were studied and finally fully understood. The vacuum losses are caused by temperature stresses around the junctions between the inner and outer tubes of the fill and vent openings of the LN2 tank. The temperature stresses were caused both by spilling of LN2 (i.e. when observing at low altitudes at certain rotator positions) and also when filling with LN2. Both the filling mouthpiece and the way the fill tube is blocked (to avoid spilling) have now been modified. So far we are using a temporary solution while waiting for some material that was ordered. Testing since December 2005 shows that we now do not have such vacuum losses. In addition, the charcoal getter is baked now at a higher temperature and installed hot in NOTCam which seems to provide a better starting vacuum. We have strong reasons to believe that we are going to be able to keep NOTCam cold using the PTR (permanent cooling unit) only during storage, compared to filling it every 3-5 days. Tests of the holding time has shown that it can last at least 10 days, but some cooling water problems (see below) which switched the PTR off has not allowed us to find the PTR stabilization temperature yet.
Thomas Augusteijn 2006-05-08