DStools

DStools is a processing pipeline to produce and post-process dynamic spectrum data products from radio interferometer visibilities. DStools currently directly supports extraction of dynamic spectra from the following telescopes:

• ATCA
• ASKAP
• MeerKAT
• VLA

Table of Contents

• Installation
• Command Line Scripts
  • ATCA Calibration
  • ASKAP Pre-processing
  • Field Modeling / Insertion
  • Model Subtraction
  • Dynamic Spectrum Extraction
  • Plotting
• DStools Library

Installation

Dependencies

DStools uses miriad for pre-processing and calibration of ATCA observations, and WSclean for imaging and model insertion. Make sure these tools are installed on your system if you wish to use these parts of the package:

• miriad
• WSclean 3.5

DStools is built on top of modular CASA which is presently only available on linux.

Installation / Configuration

Install DStools using pip or your preferred package manager:

pip install radio-dstools

Command Line Scripts

Command	Description
dstools-atca-cal†	convenience script for flagging/calibration of raw ATCA visibilities
dstools-askap-preprocess	script to set the correct flux scale and reference frame of ASKAP beams
dstools-create-model‡	script to image and model the field with WSclean
dstools-insert-model‡	script to predict model images into visibilities
dstools-selfcal	script to run a self-calibration loop using inserted model visibilities
dstools-subtract-model	script to subtract an inserted model from the visibilities
dstools-extract-ds	script to extract visibilities for use with the DStools library
dstools-plot-ds	convenience script to post-process and plot dynamic spectra in various ways

NOTE:
† miriad is a requirement to run dstools-atca-cal
‡ WSclean is a requirement to run dstools-create-model and dstools-insert-model

Below some common workflows are described. For further details on usage, run any of these commands with the --help flag.

ATCA Calibration

dstools-atca-cal is a convenience script to calibrate ATCA (CABB) data prior to use in the other scripts.

dstools-atca-cal -d <DATA_DIR> <OUT_DIR> <PROJECT_CODE>

where <DATA_DIR> is a directory containing your raw ATCA RPFITS files, <OUT_DIR> is the name of a sub-directory to store processing output, and <PROJECT_CODE> is your observation's project code. The script will by default look for PKS 1934-638 as a bandpass and flux calibrator, and attempt to identify your intended target and the corresponding gain calibrator. If you have multiple targets, an alternate primary calibrator, or want to include a polarisation leakage calibrator, you can provide their names directly with the --target, --primary-cal, --gain-cal, and --leakage-cal options.

You can provide further options (see details with dstools-atca-cal --help to:

• select the band and intermediate frequency to process,
• select the reference antenna,
• shift offsets between pointing and phase centre,
• tune calibration solution intervals,
• solve for absolute XY phase and leakage (requires a good leakage calibrator),
• produce diagnostic plots of calibration solutions,
• use the pipeline interactively, which may be necessary with heavily RFI affected observations.

The output from this script will be a calibrated target dataset in MeasurementSet format, ready for use with other scripts in this package.

ASKAP Pre-processing

ASKAP data requires extra pre-processing to

1. set the instrumental polarisation flux scale to agree with CASA conventions (e.g. I = (XX + YY)/2),
2. set the instrumental polarisation axis to align with IAU convention (X and Y receivers aligned N-S and E-W respectively), and
3. set the reference frame of the beam phase centre to the correct coordinates (by default the phasecentre is oriented to the mosaicked field centre coordinates).

These corrections should be applied before any further imaging or dynamic spectrum tasks, and are implemented using the FixMS package. You can run both steps with:

dstools-askap-preprocess <MS>

where <MS> is the path to your data in MeasurementSet format.

Imaging and Self Calibration

dstools-create-model is a script to image calibrated visibilities with WSclean and produce a model of sources in the field. See the WSclean documentation for further information on imaging modes and parameters.

Run the script with

dstools-create-model <MS>

You can supply further options (see details with dstools-create-model --help) including:

• array configuration and frequency band (to help choose imaging parameters),
• imaging phasecentre,
• mask threshold,
• clean threshold,
• robust parameter,
• maximum clean major cycles,
• maximum clean iterations,
• number of frequency sub-bands to seperately deconvolve,
• and number of polynomial terms used to model spectral structure in MFS deconvolution.

Image and model products will be stored in a directory determined by the --out-dir parameter.

dstools-selfcal is a script to run self-calibration using model visibilities stored in the MODEL_DATA column of your MeasurementSet.

Run the script with

dstools-selfcal <MS>

You can provide further options (see details with dstools-selfcal --help) to:

• solve for phase-only --calmode p or phase and amplitude --calmode ap calibration solutions,
• adjust the solution interval with -i,
• solve for gains independently over separate spectral windows with -n
• solve for either polarisation-dependent (--no-combine-pols) or polarisation-indepdendent (--combine-pols) gains.

Field Modeling / Subtraction

dstools-insert-model is a script to predict model images into visibilities stored in the MODEL_DATA column of your MeasurementSet.

dstools-insert-model <MODEL_DIR> <MS>

An interactive viewer will be launched allowing you to draw masks around any portions of the image that you want removed from the model prior to insertion. This is generally where you would remove any model components that are associated with your target of interest. Click to create polygon vertices, then either press x to mask the enclosed region or c to unmask enclosed pixels that have previously been masked. When you are satisfied with the mask close the viewer and the model images will be masked and predicted into visibilities.

dstools-subtract-model is a script that will subtract the model visibilities from the data, storing the result in the CORRECTED_DATA column of your MeasurementSet (CORRECTED_DATA = DATA - MODEL_DATA).

Run the script with

dstools-subtract-model <MS>

You can optionally use the -S / --split-data option, which will split off a new MeasurementSet (name with a .subtracted.ms suffix) with the DATA column containing the model subtracted visibilities. This can be useful for iterative model subtraction (e.g. removing bright sources far outside the primary beam prior to modelling the primary beam field).

Dynamic Spectrum Extraction

dstoools-extract-ds is the main task to extract dynamic spectra from your MeasurementSet. This command extracts dynamic spectra as a 4D cube with dimensions of baselines x integrations x channels x instrumental polarisations. Along with observation metadata this is stored as an HDF5 data structure which can be read and post-processed with the DynamicSpectrum class provided with the DStools library. By default the script will average the visibilities over the baseline axis to save memory and disk space.

Run the script with:

dstools-extract-ds <MS> <DS>

where <MS> is the path to your data and <DS> is the path to store your output dynamic spectrum.

You can supply further options (see details with dstools-extract-ds --help) to:

• set the phasecentre at which to extract the dynamic spectrum with -p <RA> <DEC> (coordinates can be in sexagesimal or decimal degree formats),
• select extraction from either the DATA, CORRECTED_DATA, or MODEL_DATA column with -d,
• throw away baselines shorter than some threshold in meters with (for example) -u 500
• disable averaging over the baseline axis with -B,
• correct for primary beam attenuation by supplying a primary beam map with -P <PB_PATH>.fits,
• disable masking of flagged data with -F.

Plotting

dstools-plot-ds is a convenience script to plot the dynamic spectra produced by dstools-extract-ds, as well as perform post-processing to produce 1D lightcurves and spectra, average the data in time and frequency, fold the data to a specified period, and perform polarisation analysis.

To produce a basic dynamic spectrum, run the script with

dstools-plot-ds -d <DS>

where <DS> is your HDF5 dynamic spectrum file.

Some other simple options (see details with dstools-plot-ds --help) include:

• choose which Stokes parameters to plot with a subset of {I, Q, U, V, L} (e.g. -s IQUV)
• plot a channel-averaged lightcurve with -l,
• plot a time-averaged spectrum with -p,
• include polarisation fraction / angle / ellipticity in lightcurve / spectrum plots with -P,
• produce a summary plot including a lightcurve, spectrum, and dynamic spectra in all polarisations with -Y,
• average in time (-t) or frequency (-f) by an integer factor (e.g. -t 5 -f 10 to average every five integrations and 10 channels),
• perform RM synthesis (using the RM-lite package) and correct for Faraday rotation with --derotate,
• provide a known RM with --rm,
• perform 2D auto-correlation of the dynamic spectra with -a to highlight periodic features,
• fold the data to a specified period with --fold -T <PERIOD>.

DStools Library

dstools can also be imported into your own scripts/notebooks as a package for more customised plotting. The main object is DynamicSpectrum which can be used as follows, supposing you extracted the dynamic spectrum to a file located at dynamic_spectrum.ds:

import matplotlib.pyplot as plt
from dstools.dynamic_spectrum import DynamicSpectrum
from dstools.plotting import plot_ds

# Create DS object
ds = DynamicSpectrum(ds_path='dynamic_spectrum.ds')

# Plot Stokes I dynamic spectrum with real visibilities and color-scale clipped at 20 mJy
fig, ax = plot_ds(ds, stokes='I', cmax=20, imag=False)

# Add or modify custom plot elements here using the fig and ax objects
...

plt.show()

The DynamicSpectrum class takes the following keyword arguments:

Parameter	Type	Default	Description
tavg	int	1	factor by which to average the data across time
favg	int	1	factor by which to average the data across frequency channels
mintime / maxtime	float	None	min and max cuts on time in units of tunit
minfreq / maxfreq	float	None	min and max cuts on frequency in units of MHz
minuvdist / maxuvdist	float	None	min and max cuts on baseline distance in units of meters
minuvwave / maxuvwave	float	None	min and max cuts on baseline distance in units of wavelengths
tunit	astropy Quantity	u.hour	time unit to use for selection and plotting
corr_dumptime	astropy Quantity	10*u.s	correlator dumptime, used to detect calibrator scan breaks
derotate	bool	False	Apply Faraday de-rotation to linear polarisations
RM	float	None	User provided rotation measure in units of rad / m^2
fold	bool	False	enable folding, must also provide period keyword
period	float	None	period on which to fold the data in units of tunit
period_offset	float	0.0	period phase offset in units of period
fold_periods	float	2	number of folded periods to display for visualisation
calscans	bool	True	insert breaks during off-source time
trim	bool	True	remove fully flagged channel ranges at band edges

Note: selection on baseline distance requires DS extraction without averaging over baselines (see dstools-extract-ds)