Quality check stage

(See also the relevant configuration parameters.)

Images are checked to ensure they meet a minimum “quality” standard such that useful scientific information can be extracted from them. If an image fails to meet quality standards, it is rejected and not included in further processing. However, it is still recorded in the database for book-keeping purposes.

The quality check code is structured such that different sets of tests can be applied to images from different telescopes (see source of the tkp.steps.quality.reject_check() function for implementation details).

Quality checks are performed on all fits images input into TraP. Two key tests are performed.

Image RMS

The central subsection of the image is iteratively sigma-clipped until it reaches a user-defined convergence giving the rms of the clipped region.

The rms is first compared to the globally acceptable minimum and maximum rms values as defined in the job_params.cfg file.

Once sufficient images have been processed (given by rms_est_history in the job_params.cfg file), the rms values for all the images are fitted using a Gaussian distribution. The rms value of the new image is then compared to the allowed sigma deviation (rms_rej_sigma) from this Gaussian distribution.

The image is rejected if the noise is signifcantly greated or less than expected.

Beam shape

The restoring beam is represented as an ellipse, parameterized by lengths of its major and minor axes and a position angle. These parameters are checked for sanity. Four separate checks are applied:

  • None of the beam parameters should be infinite.
  • Both beam axes should be at least two pixels long (the beam is not undersampled).
  • Neither beam axis should be longer than a user-defined threshold (the beam is not oversampled).
  • The ratio of the major to the minor axis should be lower than a user defined threshold (the beam is not excessively elliptical).