Overview about Code Base

VCAT consists of the two core classes: ImageData and ImageCube. Additionally, there are several helper classes that mostly run in the background. They can also be run independently, especially the vcat.helpers submodule includes several useful functions.

ImageData

ImageData represents a single observation of a radio source at a given frequency and epoch.
It accepts as input:

  • FITS files (full polarization images)
  • UVFITS (.uvf) visibility data
  • Modelfit files (.mod or .fits)

For example, loading and plotting an image can quickly be done with

from vcat import ImageData

image=ImageData("image.fits", uvf_file="data.uvf", model="model.mod")
image.plot()

Upon loading, key parameters such as image noise, integrated flux density, and fractional polarization are automatically calculated and stored as attributes.
If a modelfit file is provided, VCAT also calculates the signal-to-noise ratio (SNR), brightness temperature, and component errors automatically.

ImageData objects can be modified or analyzed using various tasks:

  • restore(), regrid(), shift(), center() – runs DIFMAP in the background to modify full-polarization images
  • get_ridgeline() – perform a ridgeline fit
  • align() – align one image to another
  • plot() – generate customizable plots of total intensity or polarization

ImageCube

ImageCube is designed to handle multi-frequency, multi-epoch data sets.
It manages a collection of ImageData objects representing different observations of the same source and provides high-level analysis methods, including:

  • Spectral analysis
  • get_turnover_map() — pixel-based turnover frequency fitting

  • fit_comp_spectrum() — component-based spectral fitting

  • Kinematic analysis

  • get_speed() and get_speed2d() — estimate apparent jet speeds and perform kinematic analysis

  • Structural analysis

  • stack() — stack multiple images

  • fit_collimation_profile() — fit collimation profiles (ridgeline or component-based)

  • Spectral & polarization mapping

  • get_spectral_index_map() and get_rm_map() — compute spectral index and rotation measure maps

  • Temporal evolution & visualization

  • plot_evolution() and plot_component_evolution() — track flux density, polarization, and EVPA evolution
  • movie() — generate interpolated movies of jet evolution in total intensity or full polarization

Configuration

Usually, the standard settings are sufficient, but some parameters can be customized via a config.yml file.
An example configuration file is provided in the repository.

To use a custom configuration file, define the environment variable VCAT_CONFIG to point to it:

export VCAT_CONFIG=/path/to/config.yml

Example config.yml

# General settings:
difmap_path: "/usr/local/difmap/uvf_difmap_2.5g/"
uvw: [0, -1]

# Method selection
noise_method: "Histogram Fit"
mfit_err_method: "Schinzel12"
res_lim_method: "Kovalev05"

# Plot settings
font: "Quicksand"
plot_colors: ["#023743FF", "#FED789FF", "#72874EFF", "#476F84FF", "#A4BED5FF", "#453947FF"]
plot_markers: [".", ".", ".", ".", ".", "."]

# Cosmology
# Values from Planck Collaboration et al. 2020
H0: 67.4
Om0: 0.315

logging:
  level: "INFO"       # Options: DEBUG, INFO, WARNING, ERROR, CRITICAL
  log_file: ""         # Optional: specify path to redirect log output to a file