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Imaging multiple channels

   

The third category of data that you might wish to image is a band of channels. That is, you wish to make a multi-frequency synthesis image and grid together all the channels in the (u,v) plane making use of the variation of u and v across the band to improve the sampling of the (u,v) plane. The output is still one image.

Both MX and HORUS can do this kind of imaging. Therefore, once again, you can either image directly from the multi-source file with HORUS or you can use SPLIT (§ 12) (you may have already done this if you have combined data from different observing runs). If you want to make a very large image you may need to sort the data into XY order with UVSRT (§ 15.2), but generally the internal sorting that HORUS and MX can do allows big enough images.

  1. Now, only small changes to the inputs of MX and HORUS as described in § 15.2 are required. For both MX and HORUS, bchan and echan specify the range of channels that will be selected, and chinc specifies the channel increment. For MX, you have independent control over the number of channels that are gridded together through the npoints adverb. Thus, you might set chinc=3 and npoints =2. MX would grid two adjacent channels in forming every output plane of the image, and the first channel in every pair gridded would increment by 3. Thus, for example, output images might be formed from channels 1 & 2, 4 & 5, 7 & 8 and so on. With HORUS, this fine control is absent. When you put optype='sum', all channels in the range bchan to echan are gridded together. I believe that chinc is ignored for this option, so that you always get one output image.
  2. Again you can either select a single IF, or you could, if it was appropriate, grid multiple IFs together as well. For example, closely spaced IFs within the same band may usefully be imaged together. Use bif and eif to select the IFs.

    In the adverb box below, I give a simple example of gridding IFs 1 and 2 and all channels in the range 8-28 together to make one output image.

MX
bif=1 Select desired IFs
eif=2
bchan=8 First channel to grid
echan=28 Last channel to grid
chinc=1 Channel increment
npoints=21 Number of channels to grid together
channel=0 No CLEAN restart

HORUS
bif=1 Select desired IFs
eif=2
bchan=8 First channel to grid
echan=28 Last channel to grid
chinc=1 Not relevant
optype='sum' Grid all selected channels and IFs

It would be interesting to compare the multi-frequency synthesis image with the channel 0 image, if you believe there is some (u,v) coverage benefit to be gained in the former.


next up previous contents index
Next: Imaging multiple freqids and Up: IMAGING Previous: Imaging one channel and

nkilleen@atnf.csiro.au