To overcome the problem of the beam changing with frequency, van Gorkom and Ekers (1983) suggested a method which models the continuum emission, Fourier transforms the model correctly for each channel (thus dealing with the frequency-dependent beam problem), and subtracts the Fourier transform from the visibilities for each channel. You then re-image the subtracted visibility data base. This procedure is quite CPU intensive, but, provided you can make a good model of the continuum emission, produces a better result than the simple image subtraction method described above, especially if the line is weak and the continuum is very strong. If the continuum emission is very simple, you may be able to model it by a collection of discrete point sources. More typically, we use CLEAN (see § 16 describing deconvolution of images) to produce this model. It is sometimes necessary to estimate a residual continuum image after this procedure and subtract it as well. This method suffers from the following difficulties.
Let us proceed with the implementation of this technique. First, you must produce a dirty image of the continuum emission with the methods presented in § 15. It may be necessary for you to image all the channels so as to ascertain which channels are line free and suitable for continuum (use ISPEC to plot spectra averaged over some spatial window or POSSM directly on the visibilities).
Next you should deconvolve the continuum with one of the CLEAN programs as described in §16 and produce a model of the continuum emission. The CLEAN model is the CLEAN component list. You must decide whether all of the CLEAN components that CLEAN found are suitable to be included in the model, or whether you should truncate the list at some point (for example, you may have CLEANed into the noise and there is not much point to Fourier transforming and subtracting noise).
The Fourier transformation of the model and subtraction from the visibilities is done with the task UVSUB. You should run this only on a single-source visibility data base. If you don't have one, go and make it with SPLIT, as described in § 12.
UVSUB | |
inname,inclass | Input visibility |
inseq,indisk | file |
nmaps=1 | One input model file |
channel=0 | Subtract from all channels |
in2name,in2class | Image associated with CLEAN |
in2seq,in2disk | component list |
outname,outclass | Output subtracted |
outseq,outdisk | visibility file |
bcomp=1 | Start subtracting CLEAN |
components at this one | |
ncomp=0 | Subtract all components ? |
cmethod= ' ' | Allow program to choose |
subtraction method | |
cmodel='COMP' | Model is CLEAN |
component list | |
factor=1 | Subtract components |
opcode=' ' | Subtract components |
smodel=0 | Used for point |
source models | |
baddisk | Keep scratch off NFS |
mounted disks |
You can now image the subtracted visibility data base with the methods described in § 15.6 to see the line-only signal.