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.
