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Hubble Space Telescope
NICMOS STScI Analysis Newsletter 40

NICMOS STAN

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| NICMOS
| STScI Analysis Newsletter
| 18 December 2008
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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
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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/

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For more detailed information and updates, please consult the
NICMOS webpages: http://www.stsci.edu/hst/nicmos

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