Incorrect information about broken tiles and antenna elements in LOFAR Measurement Sets (March 2015)

During a software roll out on Jan 26th 2015, the information on broken tiles and antenna elements, which is stored in a LOFAR internal database, was inadvertently erased. Consequently, from that moment onward the list of malfunctioning antenna elements stored in the LOFAR Measurement Sets (MS's) only contained newly diagnosed malfunctioning elements. This problem was solved on February 25th 2015. Since then, the MS's contain again the proper broken tile and element information.

Please be aware that the visibilities recorded in the MS's during the aforementioned dates are correct; only the FLAG information in the LOFAR-ANTENNA-FIELD table is wrong. Therefore, the quality of the data is still preserved.

Depending on the processing procedures applied to your data to achieve your final results, these might be affected in different fundamental ways. In particular, please note the following.

 

This issue does not affect:

  • Products of Radio Observatory pre-processing, calibrator, and target pipelines;
  • Solution transfer to the target field from calibrators observed with dedicated bandwidth;
  • Calibration results at the field center obtained adopting an external model of the sky, unless your science case requires thermal noise limited maps where the positions and fluxes of off-axis sources become important.

 

This issue affects only interferometric observations that have been processed using:

  • Products of the Radio Observatory imaging pipeline;
  • The ‘smart demix' (currently not available in the Standard Imaging Pipeline);
  • BBS (used for wide-field self calibration);
  • AWImager.

 

Since the specific effects depend on the state of the list on the observing date, as well as on the characteristics of the observed fields, it is hard to quantify for each interferometric observation performed during the last month.

The Radio Observatory could extract a history of broken and repaired antenna elements relative to the aforementioned period and consequently repair the database.

 

The script to correct your data for this issue can be downloaded from the link below:

fixbeaminfo_March2015.tar


We advise Users to compare the results obtained after correcting the MS's with those with wrong broken antenna elements information in order to estimate the impact of this issue on their scientific goals.

The software requirements to run this script on your systems are:  

1. TaQL (so casacore)

2. pyrap

3. LofIm (in particular, it uses makebeamtables and addfailedtileinfo)

 

Usage:

 

tar xvf fixbeaminfo_March2015.tar

cd fixbeaminfo

./fixbeaminfo path/to/my_ms/my_ms.MS

 

The updated number of flagged elements can be listed, e.g.,

 

taql 'select ANTENNA_ID, ntrue(ELEMENT_FLAG), [select NAME from my_ms.MS::ANTENNA][ANTENNA_ID]  from my_ms.MS::LOFAR_ANTENNA_FIELD'

 

This will give something like:

0 48 CS001HBA0

1 48 CS001HBA1

2 48 CS002HBA0

3 56 CS002HBA1

...

55 0 RS407HBA

56 0 RS409HBA

57 6 RS503HBA

58 0 RS508HBA

 

Note that in this case, it concerns the HBA dual mode, so half the tiles (24=48 polarizations) are flagged for HBA0 etc. CS002HBA1 in the above example, has HBA0 flagged (48 pols) + 4 additional tiles (8 pols) because they were set to broken.

In case you experience issues when running this script, please contact Science Operations and Support through the RO helpdesk which is hosted at https://support.astron.nl/rohelpdes

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