POL-2 data reduction fix for source blurring

POL-2 is the JCMT’s sub-millimeter polarimeter working at both 450 and 850 microns. POL-2 is a polarimeter not a detector, and so requires SCUBA-2 for use. It is used to trace the alignment of dust particles at sub-millimeter wavelengths and thus the magnetic field orientation and strength (with some additional physics added into the mix) of regions in our Universe!

Recently it has been found that sometime there is a loss of synchronisation between data values and pointing information in the data reduction process (CALCQU, run by pol2map as part of step 1). This loss of synchronisation is triggered by anomalous values in the array of HWP (Half Wave Plate) angles stored in the raw data. The result is blurring (or smoothing) of sources in some POL-2 maps (see figure below).

The fix is to download our rsync this build of the starlink software and re-reduce your data. If you look at your re-reduce data you may find that some of your maps improve, depending on whether any of your observations suffered from the blurring problem. The size of the improvement will depend on how many blurred observations you have.

For regions where multiple observations were used to produce the final maps the issue may have been less pronounced if obsweight=yes was used.

In addition, users wishing to reduce POL-2 450 micron data are asked to ensure the data have been reduced using the latest starlink 2018A software prior to this release there was a bug in the software which caused a 4 degree difference  in the angular zero point at 850 and 450, so all 450 vector maps produced so far will have a systematic error of 4 degrees in the vector angle, unless updated software (rsync starlink or 2018A starlink) was used.

The image shows two total intensity maps made from an observation of OMC1. Left: before the fix for blurring. Right: after the fix for blurring.

Also did you know you can combine various I maps into a cube to view as a movie? You can do this (assuming you ran pol2map with “mapdir=maps”) by running:

kappa

paste in=maps/\*Imap out=Icube shift=\[0,0,1\]

gaia Icube

Then in gaia, in the pop-up window that holds the cube visualisation controls, drag the “Index of plane” slider left or right to step through the planes in the cube!

You can do the same for the Q or U maps by replacing “I” with “Q” or “U” above (note, that’s an upper case “I” for the externally masked I maps – use a lower case “i” for the auto-masked I maps).

– 20180724

SMU work and data checks

In May our engineering staff undertook major maintenance work of the Secondary Mirror Unit on the JCMT. After this work it was noted that the Secondary mirror was sometimes vibrating, which lead to beam deformation. This was noticed due to sporadic increased FCF values – and could also be seen in the aspect ratio of our calibrators (see image below). Observers who collected data between UT dates May 24th 2018 and 08:10UT on June 30th 2018 should be aware of this issue. Astronomers who may have affected observations should check their data closely. This issue was noted to be intermittent. If you have questions about the data quality please contact your Support Scientist or the observatory directly.

On June 30th, we applied a temporary work-around to account for these SMU vibrations. To implement a more permanent solution, the observatory briefly removed the GoreTex membrane to work on the Secondary Mirror Unit. This work was performed between Tuesday July 24th, and Monday, July 30th.  The PI and Large Program time were unaffected.

Below is a plot showing the aspect ratios of calibrator CRL 2688 over time. The blue, shaded region represents the nominal values. Note that the high aspect ratios observed in between the temporary and permanent fix (boxed in red) were part of a low elevation, poor weather (wet grade 5), poor seeing engineering and commissioning project. Regular observing was unaffected.

 

 – 20180815

2018 OT update

We are always striving to do better at the JCMT and that means we often update our software. One software update that uses should be aware of is a new version of the JCMT Observing Tool.

It is particularly important to note that the OT version used for all semester 18B MSB preparation must be 20180516 (or newer). This may be checked via the OT start-up splash screen and also via the “Help -> About” menu option.

– 20180525