Forced Measurements¶

This page details the forced measurements obtained by the pipeline.

Definition¶

When source_monitoring.monitor: true is set in the pipeline run configuration file, any sources that have non-detections in their lightcurve will have these measurements 'filled in' by performing forced measurements. This means that the flux at the source's current average position in the non-detection image is forcefully measured by fitting a Gaussian with the same shape as the respective image restoring beam.

Forced measurements are labelled in the measurements table and parquet files by the column forced.

Minimum Sigma Filter¶

Before forced measurements are processed, a minimum sigma check is made to make sure that the forced measurements would provide useful information. For example, a dataset may contain an image that has significantly less sensitivity than the other images. In this case a faint source in the more sensitive images will not be expected to be seen in the less sensitive image. To avoid unnecessary computation, this source is not forcefully measured.

The check is performed like that which is made in the New Sources process where the signal-to-noise ratio is calculated using the rms\(_{min}\) of the image it is to be extracted from. Hence, for a forced measurement to take place the following condition must be met:

where \(i\) is the image for which the measurement is to be forcefully measured.

source_monitoring.min_sigma is able to be controlled by the user in the pipeline run configuration file. By default the value is set to 3.0.

Configuration File Options¶

The following options are present in the pipeline run configuration file to users along with their defaults:

source_monitoring:
  monitor: false
  min_sigma: 3.0
  edge_buffer_scale: 1.2
  cluster_threshold: 3.0
  allow_nan: false

• source_monitoring.monitor turns forced measurements on (true) or off (false).
• source_monitoring.min_sigma controls the the minimum sigma check threshold as explained in Minimum Sigma Filter.
• source_monitoring.edge_buffer_scale controls the size of the buffer from the image edge where forced measurements are not performed (source_monitoring.edge_buffer_scale \(\times 3\theta_{beam}\)). An error can sometimes occur that increasing this value can solve.
• source_monitoring.cluster_threshold is directly passed to the forced photometry package used by the pipeline. It defines the multiple of major_axes to use for identifying clusters.
• source_monitoring.allow_nan is directly passed to the forced photometry package used by the pipeline. It defines whether NaN values are allowed to be present in the extraction area in the rms or background maps, i.e. true would mean that NaN values are allowed.

Software - forced_phot¶

The software used to perform the forced measurements, forced_phot, was written by David Kaplan and can be found on the VAST GitHub here.