JLS Current Status

Date: 28 February 2008

TELESCOPE	Nothing to report.
SCUBA-2	The instrument has finally left Edinburgh and is on its way to the JAC. See the JCMT front page to access the STFC/UKATC press release as well as a note from the Director JAC.
HARP/ACSIS	The current status of HARP is summarised below: H03: missing from the cryostat since delivery. H14: currently unusable due to large oscillations in the mixer. This receptor has now been switched off in the data acquisition and so will appear blank in the data and real time displays at the telescope. H07: the DCMs in ACSIS for this receptor were swapped out and this has now fixed the variable Trx problem that this receptor was experiencing when it was used in multi-sub system mode. Receptors H09 and H12 are operational but have high resistance in series with the junction. For H09, this resistance has been present since the instrument was delivered. For H12, its high resistance appeared after a scheduled warm up in September. An interim solution has been to increase the bias voltage on these receptors, with minimal effect on data quality. The compressor was swapped out this week but the dewar temperature drifts (of ~ 40K over the course of a night) are still present, particularly on cold nights. This points to the cold head which will be replaced in April.
	There have been variations in calibrations since December. Investigations are ongoing to determine the cause and resolution of these. It was discovered that the temperatures of the calibration loads were at times being read incorrectly by the software, and this has now been fixed. There has been some progress in understanding the cause of the detector-to-detector gain variations. Our current understanding is summarised in the two plots below (click on the images for larger versions).
CALIBRATION
	The first graph shows the current performance of Trx for each receptor (measured over a period of an hour) at 330 GHz. The two lines shows the Trx measured for each of the two DCMs in ACSIS. You can see that some receptors (e.g. H04, H06, H10) are stable while others (e.g. H00, H02, H09) are not. One possible cause for this is the fact that the bias voltage for that receptor lies on an unstable part (i.e the slope) of the PV response curve rather than on a stable plateau. The second graph shows the effect of raising the bias voltages for each receptor by a specific, but not necessarily equal, amount. Most of the receptors which were unstable have now become more stable, although note that in some cases the absolute value of Trx has increased. Nevertheless, a stable responsivity is clearly a preference. A notable exception is in the performance of H04, which demonstrates what happens when the bias voltage is set too high - the receptor is driven to strongly oscillate. We are now undertaking a study of the receptor stability at different frequency settings across the passband of the receiver with the aim of fine tuning the bias voltages to give the most stable receptor response at all frequencies.
JLS PROGRESS	We are nearing the end of the February JLS observing block which is being hosted by the GBS. See the summit occupancy page for February 2008 to see who the observers are. Up to the time of writing, there have been a total of 36 hours available in this JLS block of which 13.4 hours have been spent observing for the JLS. Specifically, 6.1 hours were spent observing for NGS, 1.1 hours for GBS and 6.2 hours for SLS. Quality assurance of this data by the teams is now needed. There were also 8.45 hours spent observing projects from the CN queue, 2.55 hours from the UK queue and 6.7 hours on E&C work. The next observing block, hosted by NGS, is due to start in March. Note that the telescope schedule for the rest of the semester has been finalised and teams should begin to organise their observers. The current progress is shown in the bar charts below, illustrating the amount of time charged to each survey and the current completion rate. With the assumption that all the data is accepted for the survey, the NGS is more than half way through is HARP allocation.
	For reference, the JLS allocations are given here.
ARCHIVED REPORTS	An archive of previous JLS status reports is accesible from the links below: 15 February 2008 31 January 2008

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