National Radio
Astronomy Observatory

4.7.5 UV-Plane Continuum Subtraction (uvcontsub)

At this point, consider whether you are likely to need continuum subtraction. If there is significant continuum emission present in what is intended as a spectral line observation, continuum subtraction may be desirable. You can estimate and subtract continuum emission in the uv-plane prior to imaging or wait and subtract an estimate of it in the image-plane. Note that neither method is ideal, and the choice depends primarily upon the distribution and strength of the continuum emission. Subtraction in the uv-plane is desirable if continuum emission dominates the source, since deconvolution of the line emission will be more robust if it not subject to the deconvolution errors of the brighter continuum. There is also a performance benefit since the continuum is nearly the same in each channel of the observation, and it is desirable to avoid repeating its deconvolution in each channel. However, doing the continuum estimation in the uv-plane has the serious drawback that interpolating visibilities between channels is only a good approximation for emission from near the phase center. Thus, uvcontsub will do an increasingly poor job for emission distributed further from the phase center. If the continuum emission is relatively weak, it is usually adequate to subtract it in the image plane; this is described in the Image Analysis section of this cookbook. Here, we describe how to do continuum subtraction in the uv-plane.

The uv-plane continuum subtraction is performed by the uvcontsub task. First, determine which channels in your data cube do not have line emission, perhaps by forming a preliminary image as described in the next chapter. This image will also help you decide whether or not you need to come back and do uv-plane continuum subtraction at all.

The inputs to uvcontsub are:

For each baseline, and over the timescale specified in solint, uvcontsub will provide a polynomial fit to the real and imaginary parts of the (continuum-only) channels specified in fitspw (using the standard spw selection syntax), and then subtract this model from all channels specified in spw, or from all channels in spectral windows of fitspw if spw=’’. typically, low orders for the polynomial work best, like 0^th (a constant), or 1^st order (a linear fit). Use higher orders with caution and check your results carefully.

Usually, one should set solint=’int’ which does no averaging and fits each integration. However, if the continuum emission comes from a small region around the phase center and fitorder = 0, then you can set solint larger (as long as it is shorter than the timescale for changes in the visibility function of the continuum). If your scans are short enough you can also use scan averaging, solint=’inf’. Be warned, setting solint too large will introduce “time smearing” in the estimated continuum and thus not properly subtract emission not at the phase center. Increasing solint speeds up the calculation but it does not improve the overall result quality of uvcontsub - although the continuum estimates of each baseline may be noisy (just like each visibility in a continuum MS may be noisy), it is better to use the ensemble of individual fits than to average the ensemble before fitting. Note that plotms can do time and baseline averaging on the fly to help you examine noisy data.

So, the recommended procedure is as follows:

• Finish calibration as described in the previous chapter.
• 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 as low fit orders as possible to estimate and subtract the continuum from vis, and write the continuum-subtracted dataset to vis + ’.contsub’.
• Use clean with vis + ’.contsub’ to make an image cube of the line emission.
• If a continuum image is desired, clean the line-free channels of the original MS with mode=’mfs’ and spw=fitspw. Note that using the line free channels directly is preferred over the imaging the ’continuum’ model fitted by uvcontsub. The fitting procedure will also fit noise and artifacts which produce a nice line cube when subtracted, but the model may not represent the true underlying continuum.

For example, we perform uv-plane continuum subtraction on our NGC5921 dataset:

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Copyright © 2010 Associated Universities Inc., Washington, D.C.

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