Prefactor Pipeline

 

Following the standard pre-processing step performed by the Radio Observatory, direction-independent calibration can be performed using the Prefactor (v3) Pipeline.

Prefactor is a set of pipelines (Calibrator, Target and Imaging) to correct for various instrumental and ionospheric effects in both LOFAR HBA and LBA observations, as detailed in de Gasperin, F.; Dijkema, T. J.; Drabent, A.; Mevius, M.; Rafferty, D. A.; van Weeren, R., et al. 2019, A&A, 622.

It includes:

  • removal of clock offsets between core and remote stations (using clock-TEC separation)
  • correction of the polarization alignment between XX and YY
  • robust time-independent bandpass correction
  • ionospheric RM corrections with RMextract
  • removal of the element beam
  • advanced flagging and interpolation of bad data
  • mitigation of broad-band RFI and bad stations
  • direction-independent phase correction of the target, using a global sky model from TGSS ADR or the new Global Sky Model (GSM) for HBA and LBA data, respectively.
  • detailed diagnostics (and soon a summary log file)

Documentation of production branch is available here.

Prefactor data products that are made available to the user are:

  • Calibrated uv Measurement Sets
  • Calibration solutions collected in H5parm files
  • Diagnostic plots (and soon a summary log file)
  • A Stokes I full bandwidth and wide-field FITS image

 

Early Commissioning results

Commissioning of pipeline results for both HBA and LBA is ongoing. Below we summarise key diagnostics of HBA pipeline products in the image plane as obtained by inspecting a set of test fields. The table shows the range of diagnostic values obtained for the different fields. The data were recorded with typical interferometric setup: antenna mode:HBA dual inner array ; instrument filter:110-190 MHz ; time averaging factor:1 ; frequency resolution:8 channels/subband ; observation duration:4-8 h. Results for LBA data will be added in due course.


  LBA HBA (110-190 MHz)
 *Glat [deg]
   0.3-72.2
 Distance to closest A-team source [deg]
   10-33
 **Bmaj/Bmin    1.39-2.32
 Median LOFAR/TGSS flux
   0.89-1.29
 Mean RA separation (LOFAR - TGSS) [arcsec]
   -1.6 to 0.3
 Mean Dec separation (LOFAR - TGSS) [arcsec]    0.1 to 0.8
***Median inband (120-187 MHz) spectral index    -1.53 to 0.56
****RMS noise (within the main beam) [mJy/bm]
  0.5-0.9
*****Reliability [%]
   98.5-99.8

 

* No trend with Galactic latitude has been found for any of the diagnostics.

** The synthesised beam was found to be highly elliptical for some fields due to missing remote stations and/or a high amount of flagged data at the high end of the band.

*** The bandwidth was divided into 6 sub-bands. The spectral indices tend to be closer to -0.8 when discarding the highest-frequency sub-band (162-187 MHz).

**** The median RMS values for the test fields have been obtained from the RMS noise maps. Note that dynamic ranges of the order of 3500-20000 can be reached from the measured RMS values.

***** The reliability is estimated as 1.0 - Nneg / Npos, where Npos is the number of sources detected above 5 sigma and Nneg is the number of negative peaks < -5 sigma; this calculation assumes that the noise distribution is symmetric about zero.

 

Access to Data Products   

All final data products will be stored at the LOFAR Long Term Archive where, in the future, significant computing facilities may become available for further re-processing. Users can retrieve datasets from the LTA for reduction and analysis on their own computing resources or through the use of suitable resources on the GRID. More information is available here.

From the moment the data are made available to the users at the LTA the users will have four weeks available to check the quality of their data and report problems to the Observatory. After this time window has passed, no requests for re-observation will be considered.

Design: Kuenst.    Development: Dripl.    © 2020 ASTRON