Tutorial 1: Continuum Imaging

Link to Script

This script was written for CASA 6.6.4.34
Source: First Look at Imaging (CASA Guide)
https://casaguides.nrao.edu/index.php/First_Look_at_Imaging_CASA_6.5.4
Adapted for ALMA Data Processing Workshop @ University of Victoria
J. Speedie (jspeedie@uvic.ca) October 2024

Dataset: TW Hya calibrated continuum measurement set (435 MB)

twhya_calibrated.ms.tar (Download)

Your working directory should look like:

TUTORIAL1 >> ls
script_continuum_imaging.py
twhya_calibrated.ms.tar

Untar with:

tar -xvzf twhya_calibrated.ms.tar

Getting oriented with the data

listobs(vis='twhya_calibrated.ms',
        listfile='twhya_calibrated_listobs.txt')

Inspecting the data

Check the uv coverage:

plotms(vis='twhya_calibrated.ms',
       xaxis='u',
       yaxis='v',
       avgchannel='10000',
       avgspw=False,
       avgtime='1e9',
       avgscan=False,
       coloraxis='field',
       showgui=True)

Save resulting plot as png file directly:

plotms(vis='twhya_calibrated.ms',
       xaxis='u',
       yaxis='v',
       avgchannel='10000',
       avgspw=False,
       avgtime='1e9',
       avgscan=False,
       coloraxis='field',
       showgui=False,
       plotfile='twhya_calibrated.ms_uvcoverage.png')

Check the baseline coverage:

plotms(vis='twhya_calibrated.ms',
       xaxis='UVwave',
       yaxis='Amp',
       avgchannel='10000',
       avgspw=False,
       avgtime='1e9',
       avgscan=False,
       field='5', # TW Hya
       coloraxis='antenna1',
       showgui=True)

Save resulting plot as png file directly:

plotms(vis='twhya_calibrated.ms',
       xaxis='UVwave',
       yaxis='Amp',
       avgchannel='10000',
       avgspw=False,
       avgtime='1e9',
       avgscan=False,
       field='5', # TW Hya
       coloraxis='antenna1',
       showgui=False,
       plotfile='twhya_calibrated.ms_baselinecoverage.png')

Split out the science target

os.system('rm -rf twhya_smoothed.ms') # We're about to create this

split(vis='twhya_calibrated.ms', # Old measurement set (435 MB)
      field='5', # TW Hya
      width='8',
      outputvis='twhya_smoothed.ms',  # New measurement set (43 MB)
      datacolumn='data')

Inspect the new MS:

listobs(vis='twhya_smoothed.ms',
        listfile='twhya_smoothed_listobs.txt')

Image the science target

os.system('rm -rf twhya_cont.*') # We're about to create these

To auto-mask and image non-interactively:

tclean(vis='twhya_smoothed.ms',
       imagename='twhya_cont',
       field='0',
       spw='',
       specmode='mfs',
       gridder='standard',
       deconvolver='hogbom',
       imsize=[250,250],
       cell=['0.1arcsec'],
       weighting='briggs',
       robust=0.5,
       threshold='0mJy',
       niter=5000,
       interactive=False,
       nmajor = 1, # runs 1 major cycle and stops
       usemask='auto-multithresh', # Automasking parameters below this line
       noisethreshold=4.25,
       sidelobethreshold=2.0,
       lownoisethreshold=1.5,
       minbeamfrac=0.3,
       negativethreshold=15.0,
       verbose=True,
       fastnoise=False)

Alternative: Image interactively and mask manually in the GUI

tclean(vis='twhya_smoothed.ms',
       imagename='twhya_cont',
       field='0',
       spw='',
       specmode='mfs',
       gridder='standard',
       deconvolver='hogbom',
       imsize=[250,250],
       cell=['0.1arcsec'],
       weighting='briggs',
       robust=0.5,
       threshold='0mJy',
       niter=5000,
       interactive=True)

Open & inspect resulting twhya_cont* files in CARTA!

Perform primary beam correction

os.system('rm -rf twhya_cont.pbcor.image')

Perform primary beam correction and generate primary beam corrected image:

impbcor(imagename='twhya_cont.image',
        pbimage='twhya_cont.pb',
        outfile='twhya_cont.pbcor.image')