Compared with all this complicated continuum multi-frequency synthesis, spectral-line imaging is rather easy. You just need to make one image per channel, or perhaps one image per group of channels. UVMAP is not very useful for spectral line imaging, so I will just discuss HORUS\ and MX. The end result is a three-dimensional image, the third dimension being frequency. This is referred to as a cube. In spectral-line imaging, it is important that you do not make images at a frequency resolution that is much too great compared to the width of the features you are interested in. Otherwise you run the very real risk of not detecting your line. Make sure the frequency resolution is such that there are about 3-4 points across the line at most. This may require you to use the channel binning option of MX. If you are making a detection experiment, the best sensitivity to the putative line is obtained by matching the channel width to the line width.
A preceding chapter, §13 discusses other spectral-line issues, including continuum subtraction which you should do first.
All other parameters should be as described in § 15.2.
HORUS | |
Apply calibration adverbs | |
to multi-source files | |
optype='line' | Spectral line imaging of Stokes I |
smooth=1,0 | Hanning smooth |
bchan=50 | Image channels 50 |
echan=450 | to 450 at increments |
chinc=2 | of 2 say |
In the example below, pairs of channels are gridded together, starting at channel 50 and ending at channel 450. There is no smooth adverb in MX.
MX | |
in2name,in2class | Name of visibility work file, leave blank |
in2seq,in2disk | for first time, else fill in |
bif=2 | Select desired IF |
eif=2 | |
bchan=50 | Grid pairs of channels |
echan=450 | from 50 to 450 |
chinc=2 | Skip every other channel in output |
npoints=2 | Grid 2 channels together |
channel=0 | No CLEAN restart |
nfield=1 | Just one field for beginners |
fldsize=0 | For CLEAN step only |
rashift=0 | Don't shift image centre |
decshift=0 | from phase centre |
niter=0 | Don't CLEAN yet |
Last update : 27/11/93
1