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CHAPTER 4. SYNTHESIS CALIBRATION
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with names <input msname>.contsub and <input msname>.cont. BETA ALERT: be sure to
run with fitmode=’subtract’ if setting splitdata=True.
Note that it is currently the case that uvcontsub will overwrite the CORRECTED DATA column.
Therefore, it is desirable to first split the relevant corrected data into a new Measurement Set. If
you run uvcontsub on the original dataset, you will have to re-apply the calibration as described
in the previous chapter.
So, the recommended procedure is as follows:
• Finish calibration as described in the previous chapter.
• Use split to form a separate dataset.
• Use the invert or clean task on the split result to form an exploratory image that is useful
for determining the line-free channels.
• Use uvcontsub with mode=’subtract’ to subtract the continuum from the CORRECTED DATA
in the MS, and write the continuum model in the MODEL DATA column. Set splitdata=True
to have it automatically split out continuum-subtracted and continuum datasets, else do this
manually.
• Image the line-only emission with the clean task.
• If an image of the estimated continuum is desired, and you did not use splitdata=True, then
run split again (on the uvcontsub’d dataset), and select the MODEL DATA; then run clean to
image it.
For example, we perform uv-plane continuum subtraction on our NGC5921 dataset:
# Want to use channels 4-6 and 50-59 for continuum
uvcontsub(vis=’ngc5921.usecase.ms’,
field=’N5921*’,
spw=’’,
# all spw (only 0 in this data)
fitspw=’0:4~7;50~59’
# channels 4-6 and 50-59
solint=’inf’,
# scans are short enough
fitorder=0
# mean only
fitmode=’subtract’
# uv-plane subtraction
splitdata=True)
# split the data for us
# You will see it made two new MS:
# ngc5921.usecase.ms.cont
# ngc5921.usecase.ms.contsub
4.7.5
UV-Plane Model Fitting (uvmodelfit)
It is often desirable to fit simple analytic source component models directly to visibility data. Such
fitting has its origins in early interferometry, especially VLBI, where arrays consisted of only a few