Cooling

NOTCam has not been open since the 19$^{th}$ of January 2007 and the the vacuum behaviour has been very good since the vacuum leak problems were fixed. In principle, while being stored off the telescope NOTCam can be kept cold using only the PTR cooler, but in practice it is much safer to always have it filled with LN2 (which means refilling every 5-6 days) in the case of power-cuts due to bad weather or any PTR failure. An automatic warning system is now set-up with a conservative temperature limit. Even if the warnings start going off in the evening, it is safe to wait until the next morning to fill NOTCam, provided that the PTR is running well.

The maintenance of the PTR cooler (consisting in changing the compressor oil filter and the rotary valve for helium flow) has been going on since May 2006. After having received 3 rotary valve units (one wrong model, two tested but malfunctioning) our contact person at Kelvin International Corporated has arrived at the conclusion that our PTR rotary valve was fabricated according to a special design. All documentation on this unit is lost, and the only way to get it maintained is to send our working unit back to Iwatani in Japan for documentation and back-engineering. We have been told that they are capable of doing this in one week. Therefore, the rotary valve will be dismounted, packed and shipped over the period we are working on the detector and wheel bearings (see above), and a new rotary valve should be back before the end of the year.

Because it has been so difficult to do the PTR maintenance, we have been looking around at alternative solutions. The PTR solution was once selected because it has a cold head with non-moving parts with essentially infinite lifetime, provided that the oil-filter in the compressor is maintained well. There is also no need for special anti-vibration mountings, as is the case for other coolers. Prices for a system similar to the PTR are around 15000 Euros.