Search
Text Size:
Smaller Text Normal Text Larger Text

Facebook

Follow us on Twitter

NOTCam Status Report, 1999 Aug 27

Contamination in the dewar

Since the last report of February 5th, considerable work has gone into solving a critical problem - oily contamination in the dewar. 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. The HAWAII detectors, however, have an inert, sapphire cover over the detector itself and are unlikely to suffer damage from hydrocarbon contamination. (CCDs, on the other hand, can be completely ruined.)

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. Each were baked and pumped individually and in a test dewar, but no obvious source was found; even some probably non-specification greasy lubricant on the wheel bearings does not seem to have had a definitive effect.

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 present even without NOTCam connected but with just the pump and the RGA. This is an odd result because the pump is unlubricated as appropriate for high quality work.

It is entirely possible that the root cause of this problem will never be identified. The oily contaminant is undoubtedly spread throughout the dewar and particularly coats the huge surface area of anodized surfaces. The only solution, for now, appears to be to bake and pump NOTCam until no contaminants can be condensed out, slowly driving off the oil from whatever surfaces are affected. At the time of writing, this arduous process is more or less complete.

The watery contaminant is likely precisely that, water. It is a perennial problem in large vacuum systems.

Motor controller

Work on the motor controller is largely complete and all driver software checks out.

Stepper motors

The removal of all lubricants within the dewar as part of the decontamination effort increased the load on the stepper motors sufficiently that additional shims had to be introduced to reduce the pressure on the gears. Also, the focus mechanism has had to be geared down by 2:1.

Gettering experiments

As part of his degree report, French student Benjamin Noel carried out some experiments to determine the gettering effectiveness of anodised aluminium. Unfortunately, the results imply that it will have little effect. A new, large, activated charcoal getter has been designed into NOTCam.

Detector controller

Work has begun on adapting CUO controllers to run HAWAII arrays. The test dewar should be up and running within a few weeks.

Filters

The order for narrow band filters has been made to the Gemini Filter Consortium. A few have been received. Please see the filters page for complete information.

Spectroscopic facilities

The heavy delays now experienced by the project and the departure from NOT of Michael Andersen, who is the primary driver of this functionality, mean that the spectroscopic capabilites of the instrument have been temporarily shelved and the first priority is to make the instrument useful as a simple imager.

Administration

An addendum to the original NOTSA/CUO contract has been drafted in order to formalise and constrain the large amount of unforeseen but necessary work that CUO has been forced to accomodate.


Back to top Last modified: 09-Mar-2004