Gemini
Near-Infrared
Integral-Field
Spectrograph (NIFS)
First Cooldown With Mechanisms: May 2002
NIFS was reassembled for its first mechanisms test in May 2002. The
vacuum pump was switched on on Friday May 31. The cryostat
temperatures took 90 hours to stabilize to 50 K. Initial impressions
were that all mechanisms were operating correctly. This is the first
time that all the mechanisms have been tested together with an
integrated software system.
Once cold, the mechanisms were extensively tested using command
scripts. Overall performance is satisfactory. However, a few issues
have been identified.
- The grating turret operated for several days and then became
stiff and no longer turns in one direction at normal speed (250 rpm on
the motor). It does work at much reduced speed (120 rpm). The cause of
this binding is unclear.
- The OIWFS gimbal mirror does not operate at the speed at which it
was tested at the University of Hawaii (250 rpm). It can be run at 60
rpm on one axis and 120 rpm on the other axis. Determining the datum
takes 40 min at these speeds.
- An inconsistency has been identified in the drive train gear
ratios for the focal plane mask wheel and the filter wheel. Both
mechanisms use non-integer gear ratios. This means that one revolution
of the wheel corresponds to a non-integer number of turns of the drive
pinion attached to the stepper motor. The datum position is determined
from this drive pinion, so the datum position moves if the sum of the
wheel motions exceeds one complete revolution. Solutions to this bug
are being investigated.
- The detector housing temperature plateaued during the cool down
and eventually reached its expected final temperature, but at a slower
rate than expected. Warming the cold work surface plate temperature
caused an unexpected increase in the detector housing temperature. It
appears from this that the 20 K tie point is not getting as cold as
expect, possibly due to a thermal short to the cold work surface plate
where the copper shaft passes through the cold work surface plate.
- One cryocooler has developed an intermittent squeak of unknown
origin.
- Water condenses easily on the outside of the cryostat window
without the dry air supply connected.
- A thin layer of condensates has appeared on the radiation shield
and pickoff probe after two weeks of cold operation. This may be
contaminants from the manufacturing process that will eventually be
pumped out of the cryostat.
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