Imaging one channel and multiple freqids

    

Let us make the problem a little more complicated, and discuss data for which all the channels have been averaged to one (channel 0) and for which there is more than one time switched observing frequency. For example, in the 6 cm band, you may have data at 3 pairs of frequencies, spaced throughout the band, which you wish to combine to benefit from the enhanced (u,v) coverage (assuming the spectral gradients are sufficiently small).

1. Now, these data will reside in three freqids and two IFs (the simultaneous data go on the IF axis) within the one multi-source file. However, you can only access one freqid at a time. Thus, you must first split off the data associated with each freqid and IF into a separate calibrated single-source file. Apply SPLIT (See § 12) to the channel 0 file (or use the channel-averaging option in SPLIT). If you combined data from different observing runs, you will already have done this.
2. The next step is to join the files back up again ! Do this with the task DBCON. I have already discussed DBCON in § 14, and suggest you read that section. However, this time you must disable the frequency check.

   reweight=0 Leave weights alone

   dopos(1,1)=1 Check phase centres

   dopos(2,1)=-1 Don't check for frequency agreement

   doarray=-1 Separate input files in time by 5 days

3. Note that u and v are generally stored in the file in wavelengths at the reference frequency. This reference frequency will vary between the files that you care going to concatenate. However, DBCON does not recompute u and v for the new reference frequency of the output file. But all is not lost, for single channel data, the imaging tasks will not look at the reference frequency. They will simply take u and v as they are in the file so that the the image scales will come out correctly.

   Similarly, you must split the IFs separately because the concatenated file only has one FQ table, so that the second IF would not be correctly described if the 2 IFs were left together. As an example, if you had 3 freqids and 2 IFs and one channel, following SPLIT\ you should have 6 separate files, one for each freqid and IF combination. Then you join them all up again with DBCON.

4. Following all this messing about, you can now image the data with UVMAP, MX, or HORUS in the same way as described in § 15.2 above.

Again I point out that MIRIAD handles multi-frequency data in a much better fashion than AIPS.