Beam Definitions

 

A number of different types of beam are used when dealing with the LOFAR telescope. This page gives a brief summary of them.

 

Image illustrating the different types of beam used in LOFAR observations

Figure illustrating the different types of beam used in LOFAR observations. Left: Dipole or tile beam; Middle: Station beam or sub-array pointing; Right: Tied-array beams. 

 

Dipole and Tile Beams

In the LBA regime each antenna is sensitive to the entire visible sky, so the 'dipole beam' covers the entire sky.  Please note, however, that the sensitivity drops significantly below 30° elevation.

In the HBA regime, 4x4 antennas are combined with an analogue beam former before the data are sent to the station processor (RSP) boards. Each tile is therefore sensitive to a smaller portion of the sky. This 'tile beam', sometimes called the 'analogue beam', is about 30 deg (FWHM at 150 MHz).

 

Station Beam or Sub-Array Pointing (SAP)

A 'station beam', also known as a 'sub-array pointing' (SAP) is formed by combining, with the appropriate phase-rotation, all the selected antennae or (for the HBA) tiles (formally the signals  of the all selected RCUs) in a given station to form a beam towards a specified direction.

The beam-forming take place at the station processing (in the RSPs and the local computer), and is calculated one every second. This results to a gain variation of 0.3% for a  station beam of 3 deg in diameter

The beam-forming is done independently for each subband and the resulting beam of each subband is refered to as a "beamlet". Multiple beamlets with the same pointing position can be summed to produce beams with a larger bandwidth.

Up to 8 station beams, pointing in different directions, may be created, due mainly to local computing resources.  However the total number of subbands used, summed over all station beams may not exceed 488. Therefore, if 8 station beams are used with equal subbands, the total number of subbands per individual station beam should not be more than 61.

After forming a station beam towards a specified direction (say all 488 subbands in one SAP), the data flow to the CEP to be processed by the correlator.

 

Tied-Array Beams

The correlator can alternatively compute multiple Tied-Array Beams (TABs) digitally within a given station beam. The number of subbands in each TAB is equal to the number of subbands used in the station beam (SAP) and each must use an integer number of subbands. For example, if the station beam uses
122 subbands, the number of subbands in each TAB is 122.

If one has 244 subbands in a SAP, the maximum number of TABs computed is 12 for Stokes IQUV output at full resolution. Since the limitation is in terms of data rate (for computation and writing on the disk), one can have more TABs by averaging the data in time and frequency. One can compute 20 TABs if one uses only Stokes I output; again one can circumvent this limitation by suitable averaging factors in time and frequency.

If one has fewer subbands in a SAP, then the maximum number of TABs will scale inversely with the ratio of the number of subbands to the maximum of 488. 

Design: Kuenst.    Development: Dripl.    © 2014 ASTRON