CVEL errors

I'm trying to get a good image of the W51 main sources that have line forests. MFS clean does a pretty good job, but is very limited in dynamic range presumably because the spectral lines manifest as amplitude errors.

I tried splitting the data to merge my two spectral windows using cvel (code), but I see thousands of errors like this:

2015-11-02 19:52:15     WARN    SubMS::combineSpws()    Averaging failed for the following channel/correllation pairs from output row 44432 up to 44459. Corresponding visibilities will be flagged:
2015-11-02 19:52:15     WARN    SubMS::combineSpws()+   (60, 0) (60, 1) (1804, 0) (1804, 1) (1998, 0) (1998, 1)

and when I image the cube, there is a jump at one channel that occurs because the resolution suddenly changes either because SPW3 (the lo-res SPW) is not included or because the hi-res one suddenly is.

It doesn't look like there's any obvious workaround at the moment, but I'll probably try sending another helpdesk ticket about this.

My best idea for getting a decent continuum out now is something like this:

  • image the whole cube in frequency
  • determine channels that can approximate the bright continuum
  • use those to uvcontsub the data
  • image the uvcontsub'd cube
  • uvsub the imaged spectral line cube from the original data
  • re-image the now-continuum-only cube using mfs

EDIT: Dirk replied to the helpdesk ticket, suggesting:

Concerning cvel: try to process one SPW at a time. Otherwise cvel will
_combine_ the SPWs and if they are not observed at the same time, this will
lead to problems.

So I've now split the cvel into two commands. I also added outframe='bary' because otherwise it's not obvious that the frequency command is doing anything sensible or useful. That's actually probably why I was getting non-overlapping segments before - it tried converting to barycentric, but maybe too late in the process (i.e., it tried selecting in topo, then converting to bary for output). I also added a phasecenter, because it was automatically selecting pointing 24 as the phasecenter for cvel before.

This led to a new error:

2015-11-03 14:38:27     WARN    cvel::SubMS::convertGridPars     *** Requested new channel width is smaller than smallest original channel width
2015-11-03 14:38:27     WARN    cvel::SubMS::convertGridPars+        which is 488319 Hz

which I've fixed by setting the output width to be 0.5 MHz, just a simple round number greater than the input even accounting for shift. Unfortunately, this seems to have produced a dramatically larger output MS, which cannot be correct.... ahhh, it turns out that was temporary, once regridding was complete, it turned small again.

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