Major Observing Modes

Three Major Observing Modes cater for most of the astronomical needs of LOFAR users. Each of these modes has specialised sub-modes, which have their own data-analysis pipeline or use tailored pipelines connected to the end of the initial standard pipelines.

The characteristics of the modes that are offered during Cycle 13 are summarised in the following table.

A short description of each observing mode is given below and further details can be found in the relevant web pages. Other pipelines are still being developed.

 

 Type  Sub-Mode  Output  Description
 Interferometric  Correlated  Visibilities  Arbitary number of stations, 8 beams per station, full Stokes
Beam-formed  Coherent Stokes  BF data file 
 Coherent summation, core stations only, up to 219 beams (but check the beam-formed mode page for conditions), full Stokes, Stokes I or complex voltage
  Incoherent Stokes  BF data file 
 Incoherent summation, arbitrary stations, 8 beams per station, full Stokes
  Station level (Fly's eye)  BF data file
 Arbitrary stations, individual pointing and frequency settings per station, 8 beams per station, Stokes I
 Direct Storage Raw voltage  TBB data file
 Station level triggering of TBB dumps, direct storage to CEP cluster

 

Table 1: The currently supported LOFAR observing modes.  

Interferometric Mode:  

Example astronomical applications: Single Source Imaging, Surveys.

The interferometric imaging mode provides correlated visibility data (similar to traditional aperture synthesis radio telescope arrays consisting of antenna elements).  Station data streams are transferred to the COBALT correlator where they are correlated to produce raw visibility data, stored in Measurement Set (MS) format in the post-processing cluster (known as the CEP4 cluster).  

Two observing modes with associated "recipes" for the imaging pipeline are suggested for targeted observations: 

Observations with the Low Band Antennas (LBA):

Continuous in time/Hour Angle observations with half the available bandwidth on the target field (<=48 MHz, <=244 subbands) and half on a (strong source) calibrator (the same as the target<=48 MHz, <=244 subbands).

Observation in the band of 10-90 MHz.

Observations with the High Band Antennas (HBA): 

(i) Continuous in time/Hour Angle observations of the target, preceeded (and followed) by short calibrator scans or (ii) Interleaved short calibrator observations (eg. 2 min) with target field (eg. ~30 min), quasi-continuous in HA. Up to the full available bandwidth adopted. 

Observations in one of the three HBA bands: 110-190 MHz (HBA LOW), 170-230 MHz (HBA MID), 210-250 MHz (HBA HIGH)

 

Further processing of the raw uv data, which consists of calibration and imaging, is handled offline via a series of automated pipelines (see "Interferometric Modes" for a description of the Standard Imaging Pipeline).

Calibration is an iterative process of obtaining the best estimates of instrumental and environmental effects such as electronic station gains and ionospheric delays.

Final products (consisting of images and averaged calibrated uv-datasets) will be accesible to the users through the LOFAR Long Term Archive (LTA).

Further details on this mode can be found in the "Interferometric Modes" web page. 

 

Beam Formed Mode:

Example astronomical applications:  Pulsars, (exo)planets, the Sun, flare stars 

Array beams are calculated from the data streams from one or more stations in order to produce time-series' and dynamic spectra for high time resolution ( ~ μs / ms) observations. 

In the current implementation, there are three Beam-Formed sub-modes available: 

1) The Coherent Stokes sub-mode

2) The Incoherent Stokes sub-mode

3) The Fly’s Eye sub-mode

 

For Cycle 13, astronomers can propose to run observations in any of these modes.

In some cases these modes can be run as a combination.

These modes can be run in parallel with the standard imaging mode described above. This allows one to simultaneously image a field while recording high time resolution dynamic spectra to probe sub-second variations of any source in the field. More details about this commensal mode can be found in the "Commensal Beam Formed and Imaging mode" web page.

 

On-line processing of these data products via the Known Pulsar Pipeline is available during Cycle 11, producing standard pulsar dataproducts like psrfits files. There is a limited ability to create dynamic spectra. Beam-formed data products and/or Pulsar pipeline products will be stored in the LOFAR Long-Term Archive for retrieval.

More information on this mode is given on the "Beam Formed Modes" web page.  

 


Direct Storage Mode (NOTE: Expert Mode only!!): 

Example astronomical applications:  Single station all sky imaging, spectrum monitoring, intra-station baselines, local TBB experiments, detection of cosmic ray showers.  

In this mode, the station does not send its data to COBALT for correlation or beam forming. Instead the data can be recorded locally at the station computing unit or copied directly to the storage and post-processing subcluster in CEP and from there to some other facility, for further analysis.

There are two sub-modes:

1) Transient Buffer Boards:

The signal is monitored at station level by a triggering algorithm on the Transient Buffer Boards (TBBs). If triggered, the contents of the boards are frozen and sent to the CEP/OLAP for further processing.  The minimum integration time is 5 ns, and the maximum bandwidth 100 MHz. The TBBs can dump at most 5.2 sec of data per event at full resolution.
Detailed information on this sub-mode can be found in the "Direct Storage Mode" page. 

2) Single Station Observations

This is an experimental mode and not on offer for general use.

This submode records data from dipoles of the individual stations. Correlation (if needed) will be done with the station correlator, on the local computing unit.

Experimental low level software for this mode exists, as parts of the monitoring and control software. 

This mode will remain under development. 

Design: Kuenst.    Development: Dripl.    © 2020 ASTRON