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.

SCUBA-2 with a fully populated focal plane

Map of the 8 arrays in the SCUBA-2 focal plane. The data is of the responsivity of the arrays following a flatfield observation.

For several months now we have been operating SCUBA-2 with 7 of the 8 arrays – 4 at 850µm and 3 at 450µm. This is because the performance of one of the 450µm arrays, referred to as s4b, was much worse than all of the others. In troubleshooting, it was discovered that a short circuit was present on the array. During the last warm up, this short was removed by identifying the pixel responsible and then switching off the column and row it resides on. Since then we have worked  at optimising the s4b array and bringing it back into the fold. As it turns out, the s4b array is one of the better performing arrays (and there are plans to improve it further by recovering some of the lost row and columns you can see in the image above).

This is a really important milestone for the project, not least because it was never a certainty that there would be 8 working arrays, and to emerge with 8 out of 8 is fantastic. Well done to everybody involved.

Comparing array performance : one year later


Power spectrum: RED = last year BLUE = current

As we prepare to complete the array characterisation phase of the commissioning programme, we can take stock of our current performance and compare to that prior to the start of S2SRO last year.

In all metrics that we use – pixel yield, NEP, stability – we have seen significant improvements. The figure above shows a comparison of some of these for the s4a array at 450µm: red lines are data from last year with more recent measurements as blue lines. The improvements are clear and we expect yet more improvement, especially at lower frequencies (important for mapping large scale structures), once we have optimised the performance of the NTD temperature controls.

The image below depicts a figure-of-merit that is used during commissioning to aid us in optimising the array set-up. Leaving the details aside, the improvement in performance we have seen in the 450µm array compared to last year is clear (results from the other two arrays, s4c and s4d, are faded out for clarity – there is no comparison to be made for these with last year).

Figure of Merit: TOP = current ; BOTTOM = last year

This is all very encouraging news and is a testament to the work carried out last Spring/Summer on the cold electronics modules (see their installation here). But, it must be stressed that this is performance in the dark and there is no guarantee that it will map directly onto on-sky performance. However, preliminary on-sky maps taken during recent functionality tests show a qualitative improvement in performance. This will be properly quantified within the next couple of months.

First light with new arrays

GAIA screen grab of 850-micron 600 arcsec/second map of the Orion nebula (ignore the missing slice off the top of the image!)

As well as the array optimisation & characterisation commissioning that has been ongoing, we have also taken the opportunity when we can to carry out instrument functionality tests whenever possible, on the sky. This means that in the last couple of weeks we have achieved first light with the new complement of science grade arrays. Another milestone.

As well as checking focus and pointing routines, we obtained a fast map of the Orion nebula. The 850µm image above was taken at a mapping speed of 600 arcsec/second and is the output from a quick offline reduction (i.e. not optimised). Despite the instances where the telescope has overshot the mapping area, this result is particularly pleasing as it was obtained at a speed that proved to be particularly difficult for us during S2SRO. And of course, without even having started on-sky commissioning yet, this bodes well for the prospect of large mapping projects with SCUBA-2.

Improved performance with NTD temperature controllers

The image above shows the improved noise performance we see with the new NTD (neutron transmutation doped) devices which we are just now using to temperature control the focal plane arrays (there is one  NTD device per focal plane). The data show sequential noise measurements made with the instrument shutter closed (i.e. in the dark) for 6 of the arrays in SCUBA-2. Sequences spanning 1 to 120 show the measured noise without the NTD control loop switched on, and sequences 121 to 250 with it switched on (ignore the large spike around sequence 130 which is likely due to “settling time issues”)

Use of the NTD control loop to stabilise the temperature of the focal planes has a clearly significant and beneficial effect on the stability of all of the arrays over the course of the 6 hours that these test data were acquired.  Similar tests were repeated on two subsequent nights and showed the same benefit from having the NTD temperature control of the focal planes switched on.

Tc flattening

Before (left) and after (right) images showing the effect of Tc flattening on SCUBA-2 arrays.

This past week has been a busy one for the SCUBA-2 commissioning week. Using a technique, which we are calling “Tc flattening”, the team have shown that it is possible to return a larger number of bolometers in an array into transition. There are two issues: 1) the transition temperature of the bolometers varies slightly across the arrays, and 2) there is a small temperature gradient across the arrays. Combined, this means that for a given temperature of the focal plane arrays, although most of the bolometers will be in transition, some will be too warm and they will go normal, whilst others will be too cold and they go superconducting. In either case, those pixels are unresponsive. To determine the optimum setup that returns the highest yield of bolometers in transition is one of the main tasks of this phase of commissioning.

By turning on one of the SQUID bias supplies during an otherwise quiescent part of the readout cycle, it is possible to apply a different amount of heat to each row to counteract and flatten the variation in Tc and temperature across the arrays – we have dubbed this process “Tc flattening”.

The image above shows a first result of using this technique on array s8b (one of the 850µm arrays). The image on the left shows the number of usable pixels before Tc flattening and the image on the right shows the same after Tc flattening has been applied. We managed to get nearly 200 more bolometers into transition with this technique and we hope for more with further improvements.

SCUBA-2 is back to base temperature

Happy New Year to the JCMT community! The Christmas and New Year break came as a welcome respite for the SCUBA-2 commissioning team here at the JAC, the UK and Canada. During that time, SCUBA-2 was warmed back up to ambient temperatures, also providing team members time to analyse the commissioning data obtained up to now.

Analysis of this data is encouraging and there are signs in the data which suggest that these arrays will perform significantly better than we saw during the SCUBA-2 Shared Risks Observing campaign in February-March 2010. There is still work to be done in optimising the array performance and this is what the commissioning team will be concentrating on for the next month. After that, it will remain to be seen whether the improved performance characterstics we are seeing in the dark (i.e. with the instrument shutter closed) will translate to on-sky performance.

The instrument was cooled back down to base temperature last week with no issues or problems.