Spectral-line case

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Next: How to interactively define one or more CLEAN Up: Making Images Previous: Continuum polarimetry case

Spectral-line case

For the spectral-line data set, total intensity images of the HI in the galaxy NGC 5921 are generated for each channel to form a spectral-line cube. In this example, the CLEANing process is done on the line+continuum data and then the continuum is subtracted in the image plane.

Cleaning a spectral-line cube with the continuum in each channel can be inefficient if most of the clean cycles are spent cleaning the same structure (e.g. continuum emission) in each plane. Thus, it is sometimes better to subtract the continuum emission before the CLEAN process. A Glish script along with a description of how to do continuum subtraction in the uv plane, before imaging, will be available soon.

CLEANing a cube is similar to CLEANing a single continuum channel except that a mask can be defined for each channel. The mask can be a single shape that is defined for all channels (e.g. as in the continuum polarimetry example) or it can have a variable shape depending upon the location of the emission in each channel. In the simple example below, the image data, parameters, and weighting function are defined first. The inner quarter of the cube (continuum + line emission) is then CLEANed for 3000 iterations:

imgrS:=imager(filename='ngc5921.ms');# Create imager tool if not already done. 
imgrS.setdata(mode='channel',        # Select channel data for field 3
              nchan=60,
              start=3,
              step=1,
              fieldid=3);
imgrS.advise(takeadvice=F,           # Determine image and cell size using
            fieldofview='60arcmin'); #  advise function.  Do not take advice,
                                     #  note parameters in logger
                                     #  window and use for setimage.

The Advice function will produce logger messages which you can use to guide your choice of image properties:

  Advising image properties
  Maximum uv distance = 4798.32 wavelengths
  Recommended cell size < 21.4934 arcsec
  Recommended number of pixels = 180
  Dispersion in uv, w distance = 1928.57, 788.584 wavelengths
  Best fitting plane is w = -0.253496 * u + -0.574818 * v
  Dispersion in fitted w = 178.7 wavelengths
  Wide field cleaning is not necessary

Set the image parameters and weights:

imgrS.setimage(nx=256,               # Imaging parameters
               ny=256,
               cellx='15arcsec',
               celly='15arcsec', 
               stokes='I',          
               mode='channel',
               nchan=60,
               start=3,
               step=1,
               fieldid=3);
imgrS.weight(type='briggs',          # Robust weighting
             rmode='norm',
             robust=0.5);

To examine the synthesized beam or Point Spread Function to determine the resolution resulting from the chosen weighting scheme and see what artifacts to expect from beam sidelobes during CLEANing (Fig. 1.20):

imgrS.makeimage(type='psf',          # Form the PSF image if desired
                image='ngc5921.psf');
imgrS.fitpsf(psf='ngc5921.psf');     # Measure the beam size

The imgrS.fitpsf function will report the FWHM of the synthesized beam in the logger:

    Beam fit: 53.2801 by 45.8792 (arcsec) at pa -164.714 (deg)

$\begin{figure} % latex2html id marker 1180 \epsfig{file=cookbook.dir/vla.psfS.ps... ...plotted at -5, 5, 10, 30, 50, 70, \& 90\% of the peak.} \hrulefill \end{figure}$

To Hogbom CLEAN the inner quarter of the image down to a threshold level of about the expected RMS level in a single channel (2 mJy):

imgrS.clean(algorithm='hogbom',      # Image and deconvolve inner quarter
            niter=6000,              #  with Hogbom CLEAN down to a threshold
            gain=0.1,                #  of 2 mJy
            threshold='2mJy',         
            model='ngc5921.mod',
            image='ngc5921.im',      #  Write the cleaned image to the file
            residual='ngc5921.resid');#  ngc5921.im on disk.
dv.gui();                            # Bring up default viewer tool to 
                                     #  examine final image.

Use the viewer to display the image ngc5921.im and obtain statistics (Fig. 1.21). The final image should have a peak of $\sim$ 89.2 mJy beam^-1 and an RMS $\sim$ 1.3 mJy beam^-1 in each channel. You should see several continuum sources and the HI emission in the galaxy as you move through the cube.

**Figure 1.21:** Line+continuum emission at 6 cm in channel 40 of the NGC 5921 cube. Grey scale is plotted from 0 to 89.3 mJy beam^-1, the RMS is 1.3 mJy beam^-1, and contours are plotted at -3, 3, 5, 10, 20, 30, 50, & 70 $\sigma$ . The synthesized beam is shown in the bottom left corner of the image.
$\begin{figure}\epsfig{file=cookbook.dir/vla.ngc5921.image.ps, height=3in}\hrulefill \end{figure}$

Next: How to interactively define one or more CLEAN Up: Making Images Previous: Continuum polarimetry case Contents