On baking and pumping the dewar, some oily condensate has been observed, particularly on the cap covering the PTR port and the valve to the pump. These are two of the coldest regions of the dewar during baking and are reasonable sites for condensation of contaminants which are evaporated from the hotter parts. A cold trap placed on the pump line also accumulated considerable condensation but apparently mostly water based although not completely clear and of a yellowish tint.
Contamination is a potential serious issue in a dewar.
This is because it can change the optical properties of lenses and filters and, in some cases, permanently compromise the sensitivity of the detector. In an effort to trace the source of this contamination, considerable effort was put in by Anton and Benjamin over the summer, removing and cleaning the various components, especially bearings. A residual gas analyser was purchased in July 1999 and sent to CUO to be used to analyse the composition of the best vacuum that could be acheived. The only anomalous result was the discovery of a considerable proportion of mechanical pump oil in the system. This is an odd result because the pump is unlubricated as appropriate for high quality work.