LOFAR can return simple time/frequency beam-formed data instead of, or combined with, interferometric data. 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 and frequency resolution applications. Typical applications include pulsars and solar or planetary studies.
The minimum integration time is 5.12 μs. The maximum spectral channel
the width of one sub-band (195.3125 kHz with the 200 MHz clock).
Sub-bands may be split into a number of channels (this number must be a power of 2, so usual choices are 16 channels, 64 channels, 256 channels, up to a maximum
of 2048), to provide higher
spectral resolution. Increasing the spectral resolution
comes with a corresponding increase in the minimum integration time: This is calculated by taking the inverse of the frequency
resolution. For example, if 256 channels per sub-band are specified,
the minimum integration time will increase to 0.0013s.
In the current implementation, there are three Beam-Formed sub-modes which can be used individually or combined in the same observation:
1) The Coherent Stokes (CS) sub-mode produces a coherent sum of multiple stations (also known as a “tied-array” beam) by correcting for geometrical and instrumental delays. This produces a beam with restricted field-of-view, but with the full cumulative sensitivity of the combined stations.
This sub-mode is currently restricted to observations using only core stations; stations outside the core do not use the same clock and are not fully phased-up to the core stations.
Up to 191 simultaneous, full-bandwidth tied-array beams with different pointings within the station beam of a single core station are currently supported in this mode.
2) The Incoherent Stokes (IS) sub-mode produces an incoherent combination of the various station beams by summing the powers after correction for only the geometrical delay. This produces beams with the same field-of-view as a station beam, but results in a decrease in sensitivity compared with a coherently-added tied-array beam.
One such incoherent array beam can be formed for each of the specified station beams - i.e., if all the stations being summed split their recorded bandwidth across say 8 pointing directions, then 8 incoherent array beams can be formed from these.
All LOFAR stations, including the international stations, can be summed in this sub-mode.
3) The Fly’s Eye (FE) sub-mode records the individual station beams (one or more per station) without summing. This is useful for diagnostic comparisons of the stations and other applications where station data need to remain separate. In combination with the Complex Voltage sub-mode, Fly’s Eye can be used to record the separate station voltages as input for oﬄine processing, but be aware that the data volumes for this are large and so the number of stations which can be included in such an observation is limited.
Polarizations (CS/FE), Polarizations (IS):
For the different modes explained above, polarizations can be selected:
Combining Interferometric and Beam Formed Modes
The above modes, and in some cases even a combination of these modes, can be run in parallel with the standard imaging mode described in a separate section. 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.
Please note however, that all stations are added to the beam-forming (currently it is not possible to select a subset for processing of the beam-formed data). In particular for the Coherent Stokes, it is therefore important to use only core stations for combined observation.
Pulsar observations may be processed via the Known Pulsar Pipeline, as given in the following schematic and described in more detail by Stappers et al (2011).
A schematic overview of the overall Pulsar Pipeline, as it
runs online on the Correlator, followed by offline scientific processing on
the offline cluster. Offline pipeline processing can be run on data
directly out of the Correlator or on RFI-filtered data.
The Beam-Formed data written by the Correlator are stored on the LOFAR offline processing cluster in the HDF5 format (Hierarchical Data Format). Several conversion tools have been developed to convert these data into other formats, e.g. PSRFITS, suitable for direct input into standard pulsar data reduction packages, such as PSRCHIVE, PRESTO, and SIGPROC.
Among other things, these reduction packages allow for RFI masking,
dedispersion, and searching of the data for single pulses and periodic
signals. Coherent dedispersion can be carried out
online, also for multiple beams/dispersion measures. Online
RFI excision to excise corrupted data
from individual stations before they are processed in the correlator is under development but not offered for Cycle 2.
Currently, the known pulsar pipeline is only able to operate on Stokes I or Complex Voltage data only. Operation of the pipeline for Stokes IQUV data is under development.
This pipeline cannot be run automatically after an observation and so is only available for use with pulsar observations.
Since the Beam-Formed data serve a much larger community than pulsar astronomers, a dynamic spectrum tool is under development. This tool allows for the creation of dynamic spectra from the beam-formed data files and includes some functionality to re-bin data in time and/or frequency. It also includes the ability to only retain a useful part of the original data. Thus a user could use this tool to obtain a quick, low resolution, look at the data to identify regions of interest and then retain only these, discarding remaining, redundant, data. All dynamic spectra, whether processed or not, are stored in an HDF5 file format.
During Cycle 2, we may be able to offer *strictly limited* use of this tool upon request. If use is granted, the Observatory can run the tool to generate quicklook plots for the user to review and send their specifications for running the full tool to Observatory staff, who will then run the full tool *only once* per observation. No re-run will be permitted in the event of failure and multiple runs on the same dataset cannot be accommodated. All non-pulsar data will be made available to the user in their raw form via the Long-Term Archive. Please also note that, currently, there are no RFI excision tools available for these data.
Table 1 gives an indication of sensitivity and processing speed of typical Beam Formed observations.
Table 1: Sensitivity and Processing Performance Parameters for typical Beam-Formed Observations, assuming 16-bit mode with a maximum of 244 sub-bands.
1 This assumes the raw 32-bit floats written out by the Correlator. The Known Pulsar Pipeline converts these to 8-bit integers, and hence reduces the data volume by a factor of 4 in the case of Pulsar observations. If the scientific goal is to create dedispersed/folded pulse profiles, then the volume of the resulting data products is over an order of magnitude smaller. The dynamic spectrum pipeline may also do this, once developed.
2 Approximate minimum, period averaged, 100 MHz flux density for a detection with a signal-to-noise ratio > 10. For Coherent Stokes, this assumes the 6 Superterp stations. Using all Core stations, the raw sensitivity increases by a factor of 4 (i.e. the minimum flux becomes ~3mJy. For Incoherent Stokes, 48 incoherently added HBA sub-stations (24 tiles) are assumed. For Fly's Eye the sensitivity corresponds to that of a single HBA sub-station.
3 Processing time includes: a) conversion of 32-bit to 8-bit data; b) RFI flagging; c) dedispersion; d) folding; e) creation of diagnostics plots. Multiple beams/stations can be run in parallel. Standard observations of known sources often require only 1 beam.