Data preparation

Rapthor requires that the input data be prepared using the LOFAR initial calibration (LINC) pipelines. These pipelines perform the calibration of the calibrator data, the removal of instrumental effects (e.g., station clock offsets), the setting of the overall amplitude scale, and the direction-independent calibration of the target data. The pipelines are available at https://git.astron.nl/RD/LINC and must be run before Rapthor can be used.

Input measurement sets

The input data must have the direction-independent solutions applied to the DATA column (this is provided already by LINC). The multiple frequency bands output by LINC can be input directly to Rapthor (no concatenation is needed). Data from multiple epochs, such as interleaved observations or observations from multiple nights, are supported.

Sky model

Manual sky model input

If self calibration is to be done (see Setting the processing strategy for information about defining a processing strategy), Rapthor requires an initial model to start the calibration of the first iteration (after the first iteration, Rapthor will use the model that is the result of the previous iteration). If such a model is available, it can be specified using the input_skymodel option in the parset. Typically, however, an initial model is not available, in which case Rapthor can either generate the model (recommended) or download one. See below for more information on these options.

The input sky model consists of a list of sources stored in a plain text file, their coordinates, flux density, and other relevant parameters. For the curious reader, an overview of the data format is available in the WSClean manual.

Automatic sky model generation

If an initial sky model is unavailable, Rapthor can generate the sky model automatically from the input data. The generation can be activated by setting generate_initial_skymodel to True in the parset. Rapthor will then image the full field and generate the initial model from the resulting clean components.

This method will usually result in a higher-quality sky model than can be obtained through downloading a model from a catalog. Additionally, although the generation of the model typically requires much more time than the download, the higher quality of the model means that many of the early self calibration cycles can be skipped, resulting in overall less time being required for a full reduction.

Note

The default “selfcal” strategy will skip the phase-only self calibration cycles if automatic sky model generation is used. See Setting the processing strategy for details.

Automatic sky model download

Rapthor can also download a suitable sky model automatically. If download_initial_skymodel is set to True in the parset, rapthor will use the additional download_initial_skymodel_radius and download_initial_skymodel_server to download a sky model out to the given radius from the given source. See The Rapthor parset for more explanation about these parameters.