Vacuum & Cooling

When NOTCam was set to warm up on December 13th for a planned opening, it had been running cold for 19 months, the longest time ever. This is the result of an excellent vacuum obtained from 1) the new entrance window installed March 2008, 2) an improved LN2 filling nozzle, 3) an improved alert system, and 4) proper baking of the entire cryostat in March 2008. We therefore decided to go for a long-term baking also in January 2011, but in order to streamline things and reduce the work load, we selected to do only a one-stage baking with the radiation shield in. In hindsight this was probably not a good idea since the vacuum obtained when pumping was not as good as back in 2008. For the future we know that a two-stage baking, first with the radiation shield removed, and then with the shield back in place, though more laborious, gives better vacuum.

The opening in January was reduced to a minimum of days, and special care was taken with respect to cleanliness and statics all the time when touching the instrument.

The minor problem with faulty readings of the cryostat pressure and four temperature detectors suddenly disappeared by itself after NOTCam was opened. Since NOTCam was back in the dome early February we have not seen any faulty readings.

Since the Helium leakage of the hose of the PTR cooler was fixed in June 2010, we have seen no problems, i.e. no new leaks. Care is taken to treat the hoses well when moving NOTCam across the floor, but some wear and tear is expected as these are constantly moving around when mounted on the telescope. We got quotes for a set of new hoses, but in the past we have been able to simply repair the existing one and we have still to decide if we buy new hoses as a set of spares.

As part of maintenance of the PTR it is needed to exchange the oil filter in the compressor as well as the rotary valve for the cold head. For the rotary valve we have a spare and in principle we can just run the existing one until it stops and replace it. The oil filter was refurbished in 2006 when the system had a total running time well beyond the recommended time for exchanging the filter, while the total running time since then is even slightly longer. It is not entirely clear how necessary the exchange of the oil filter is, but it is mentioned that it might otherwise damage the cold head. Also given the fact that we attempt to keep NOTCam cold as much of the time as possible which means high running time ($\sim$8000 hours a year) we decided to refurbish the filter. Normally we would have to send out our current filter, but we have now an offer from the company to supply us with a replacement filter before we send our current filter so that we can exchange the filter without having to stop the cooling for any extended period of time.