Cross-calibration

[relevant workers: crosscal, inspect]

Cross-calibration runs largely on CASA tasks. Using these tasks, CARACal allows users to solve for delays, bandpass, gains and flux scale in several different ways. The crosscal worker operates within the framework that .MS files include a primary (bandpass and flux) calibrator and, optionally, a secondary (gains) calibrator.

Just as a classic, simple example, it is possible to solve for:

  1. time-independent antenna delays and normalised bandpass based on the observation of the primary calibrator;
  2. time-dependent antenna flux scale based on the observation of the primary calibrator;
  3. time-dependent antenna gains based on the observation of the secondary calibrator;
  4. time-dependent antenna flux scale at fine time resolution obtained by scaling the gains from step 3 above to the gains from step 2 above.

However, CARACal allows users to take less traditional cross-calibtation steps, too, such as self-calibration on the secondary calibrator, or delay calibration on the secondary, and to flag the calibrated visibilities in between calibration steps.

Flexible cross-calibration strategies

CARACal allows for powerful and sophisticated cross-calibration strategies thanks to the flexibility provided by the parameters crosscal: primary: order and crosscal: secondary: order. These allow users to build their favourite sequence of calibration/imaging/flagging steps choosing among:

  • K = delay calibration with CASA GAINCAL
  • B = bandpass calibration with CASA BANDPASS
  • G = gain amplitude and/or phase calibration with CASA GAINCAL
  • F = gain amplitude and/or phase calibration with CASA GAINCAL, followed by bootstrapping of the flux scale from the primary calibrator with CASA FLUXSCALE (secondary calibrator only)
  • I = imaging with WSCLEAN (secondary calibrator only)
  • A = flagging with CASA FLAGDATA using the tfcrop algorithm

Each of these steps may have its own settings with respect to gain type (e.g., each G could be amplitude-only, phase-only, or both amplitude and phase), solution interval, normalisation, data combination at boundaries (e.g., scan, SPW), imaging and flagging settings.

For example, crosscal: primary: order: KGBAKGB results in:

  • delay calibration (K);
  • gain calibration (G) applying the intial K on the fly;
  • bandpass calibration (B) applying the initial K and G on the fly;
  • flagging of the visibilities with the initial K, G and B applied;
  • final K calibration applying the initial G and B on the fly;
  • final G calibration applying the final K and initial B on the fly;
  • final B calibration applying the final K and G on the fly.

In this example, it would be possible to set different solution intervals for the initial and final G through the crosscal: primary: solint parameter, which is a sequence containing one entry per element in crosscal: primary: order. In case the solution interval is not relevant (A and I steps) users can give an empty string ‘’. The same applies to the calibration parameters crosscal: primary: calmode and crosscal: primary: combine.

An example for the secondary is crosscal: secondary: order: FIG, which results in:

  • gain calibration and bootstrapping of the flux scale;
  • imaging;
  • gain calibration.

Note that in this example no bootstrapping of the flux scale is necessary after the second gain calibration G because the gains are now self-calibrated on a I sky model which, following the initial F, is already on the correct flux scale.

We refer to the crosscal page for a complete description of all cross-calibration parameters.

Apply the cross-calibration and diagnostic plots

CARACal can apply the cross calibration tables to all calibrators (useful for diagnostics). It can also apply it to the target, although this can also be done by the transform worker on the fly while splitting the target from the input .MS file. When applyin the calibration, the crosscal worker adopts the following interpolation rules:

  • Delay calibration: applied to primary, secondary, target with nearest, linear, linear interpolation, respectively.
  • Bandpass calibration: applied to primary, secondary, target with nearest, linear, linear interpolation, respectively.
  • Gain calibration before bootstrapping the flux scale: applied to primary, secondary, target with linear, linear, linear interpolation, respectively.
  • Gain calibration after bootstrapping the flux scale: applied to primary, secondary, target with linear, nearest, linear interpolation, repsectively.

The crosscal worker produces .HTML plots of the various calibration terms for later, interactive inspection. Furthermore, the inspect worker produces .PNG plots of the caibrators’ calibrated visibilities to check the quality of the calibration. A variety of standard plots are produced, such as phase-vs-uvdistance and real-vs-imaginary. Furthermore, users can define their own plots as described in the inspect page.

We strongly recommend that users inspect the .HTML and .PNG plots produced by the crosscal and inspect workers to ensure that the quality of the cross-calibration is adequate to their science goals.