SCUBA-2 warm after 8-months of continuous operation


It doesn’t seem that long ago when one of our major concerns with the SCUBA-2 project was the reliability of the fridge. In March, earlier this year, we installed a liquid Helium cold trap with the aim that it should filter out any contaminants that may have inadvertantly entered the He/He-3 gas mixture.

It’s astonishing how well this has worked. As the graph above shows, the fridge has kept its temperature all the way through the last 8 months, allowing us to control the temperature of the arrays at about 0.09 K. Furthermore, consider that the fridge was designed to only ever be cold for 6  months at a time.

So just this week, we initiated a controlled warm up of SCUBA-2 so that we can carry out some essential maintenance on the refrigeration system. We will be cooling the instrument back down in a few weeks and be ready for more observing with SCUBA-2 at the start of December.

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Mauna Kea earthquake ‘felt’ by SCUBA-2

Last Wednesday on the 19th of October, just after 2pm, a 4.5 magnitude earthquake shook the Big Island. These earthquakes are not an uncommon occurrence here, with frequent tremors occurring of around a magnitude of 4 and below. However, this particular earthquake happened just to the west and about 19 km below Mauna Kea and was felt by other observatories on the summit, including UKIRT where the shear pins, which protect the telescope structure from violent shakes, had to be replaced.

JCMT escaped unscathed, but the tremor was detected in the SCUBA-2 temperature logs, as can be seen in the graph above. The units on the left-hand axis are in degrees Kelvin. The mechanical energy that entered the instrument due to the earthquake was registered as a small temperature rise in the mixing chamber of SCUBA-2’s dilution refrigerator. This change in temperature was just 20 mK, or 0.02 degrees.  There were a number of aftershocks that followed, and these were also detected by the temperature sensors in the fridge, but these are harder to spot on this graph.