NICMOS STAN +----------------------------------------------------------+ | NICMOS | STScI Analysis Newsletter | 18 December 2008 +----------------------------------------------------------+ Visit the NICMOS web site http://www.stsci.edu/hst/nicmos "New in the Last 45 Days" for all new information about NICMOS. CONTENTS: NICMOS / NCS Cooldown Restart +----------------------------------------------------------+ NICMOS / NCS Cooldown Restart As of 1:20pm EST Tuesday 16 December 2008, the compressor on the NICMOS Cooling System / NICMOS Cryo-cooler (NICMOS NCS/NCC) was restarted and the system has since started cooling successfully, with NICMOS expected to be operational in a few weeks from now. This concludes several restart attempts during the past few months, which began initially after the NCS/NCC had to be shut down for the spacecraft computer (NSSC-I) update in September 2008. During the first NCC/NCS restart, onSeptember 11, the system ran for several hours but with clearly anomalous signatures, before safing due to an overspeed violation of the circulator motor which is responsible for moving the coolant gas through the system (while a different motor, the compressor, is responsible for cooling the gas). Three more restarts of the circulator motor were attempted between September 11 - 14, 2008, but none of these were successful. One hypothesis suggested that small amounts of icy contaminants, possibly water ice, may have become dislodged as a result of thermal variations or disruptions during the initial NCC shutdown and restart process, and that these contaminants may have been transported from relatively benign areas to the location of the circulator pump mechanism and sensors, where they may have then interfered with the nominal operation of the system. The system was therefore allowed to warm up for a sufficient length of time to enable the circulator pump housing to achieve temperatures above the freezing point of water, so that when the circulator pump was restarted the contaminants would then be more easily removed. This thermal state was reached by around the second week of December 2008, and some additional few degrees of heating to the circulator pump was provided by stalling the CPL (capillary pumped loop) for a few days, which otherwise would normally remove heat from the NCC. By Monday Dec 15 2008, the circulator pump had warmed to about 10 C, sufficient to ensure that the contaminants would be thawed if indeed they consisted of water ice, and the decision was made by HST Project to restart the circulator pump motor. In addition, other parts of the system still remained much colder; in particular the VCS (vapor cooled shield) was still at around 215 K, and would thus serve as a trap for the hypothesized contaminants, allowing them to deposit in a more benign location than around the circulator pump. This time the circulator pump was able to continue running and achieved a stable current of 67 mA for over 8 hours, much closer to its nominal value, hence HST Project recommended on Tuesday 16 December to proceed with turning on the compressor pump in order to start cooling the system. The cooling has been proceeding nominally since Tuesday 16 December, and it is expected that NICMOS should achieve operating temperatures some time in January. As the cooldown proceeds, the time at which operations can begin will become known progressively more accurately. An announcement will be sent at the point when NICMOS reaches a stable operating temperature, and until then, updates on the cooling progress will be posted on the NICMOS webpage: http://www.stsci.edu/hst/nicmos/ +----------------------------------------------------------+ For more detailed information and updates, please consult the NICMOS webpages: http://www.stsci.edu/hst/nicmos +----------------------------------------------------------+ | Help: help@stsci.edu +----------------------------------------------------------+ | To subscribe or unsubscribe to the STAN, send a message | to majordomo@stsci.edu with the subject line blank and | the following in the body: [un]subscribe nicmos_news +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
