Major Observing Modes

Three Major Observing Modes caters 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 2 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, Superterp only, 127 beams, full Stokes
  Incoherent Stokes  BF data file 
 Incoherent summation, arbitrary stations, 8 beams per station, full Stokes
  Complex Voltage   BF data file  Coherent summation, Superterp only, bypasses second poly-phase filter, raw voltage output 
  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 Modes:  

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 beams are transferred to the Central Processing (CEP) facility (during Cycle 2, COBALT will be used as a correlator) where they are correlated to produce raw visibility data, stored in Measurement Set (MS) format in the post-processing cluster (known as the CEP2 cluster) also located at CEP.  

In LOFAR Version 2.0, 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 10-80 MHz.

Observations with the High Band Antennas (HBA): 

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 two HBA bands: 110-190 MHz (HBA LOW), 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).

After a proprietary period, the final producs will become publicly available. 

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, cosmic rays.  

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 observations. 

The minimum integration time is 5.12 μs. The maximum spectral channel width is the width of one sub-band (195.3125 kHz with the 200 MHz clock). Sub-bands may be split into multiples of 16 channels, up to a maximum of 256 (though more can be specified in Expert Mode), to provide higher spectral resolution.  Please note that increased spectral resolution comes with a corresponding decrease in time resolution: The time resolution is calculated by taking the inverse of the frequency resolution (for example, 256 channels per sub-band yields a maximum time resolution of 0.0013s.  

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

1) The Coherent Stokes sub-mode

2) The Incoherent Stokes sub-mode

3) The Complex Voltage sub-mode

4) The Fly’s Eye sub-mode

 

For Cycle2, astronomers can propose to run observations in any of these modes.

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

In Expert Mode, 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. This combined functionality will likely not be available at the beginning of the cycle.

Beam-formed data products will be stored in the LOFAR Long-Term Archive and may be retrieved by investigators from there.  On-line processing of these data products via the Known Pulsar Pipeline or to create dynamic spectra will be available later.  

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

 


Direct Storage modes (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 back to COBALT for correlation or beam forming system. 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 1.3 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 in LOFAR v. 2.0

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.    © 2014 ASTRON