[evlatests] VLA image -- and an interesting clue ...

[Michael Rupen]

Wow, everyone's been very busy!  Great stuff...


> 4. Gain calibration was determined (using the bandpass obtained above) from
> the on-source scans both per-integration and per-scan.  The per-integration
> solutions reveal considerable time-variation in the phase (up to ~rad) on
> timescales shorter than the scans; the long-timescale phase variation is much
> smaller.  Closely spaced antennas show very similar phase fluctuations, as
> expected.

I assume these fast variations are seen on the simultaneous VLA data as
well? Maybe Rick can comment.


> 5. Both the bandpass and gain calibration were applied to all of the data
> (the gain calibration differently for the two fields as noted below).  I had
> no trouble calibrating the bulk delay (cf Ed), but I note that the constant
> delay calibration performed here leaves behind a residual of a few 10ths
> of a nsec that fluctuates on timescales of a few seconds to minutes.  This
> residual is nominally too small to explain the freq-dep effects discerned
> from the images and described below.

This is related to a point Ken has often made: the residual delay clunking
we see with WIDAR is much less than the residual delay clunking with the
VLA correlator, where we don't notice them because the dump times are long
and the frequency resolution is poor.  If delay clunking were a killer we
would have much worse problems with the VLA correlator.  Ken, could you
send out a brief summary of this for the VLA compared to the WIDAR correlator?
I think at this point that would be useful to have written down for all of us
to ponder.


> 6. 0217 on-source imaging:  Calibrated with the per-scan gain solution, the
> dynamic range is ~500 per 1 MHz channel and ~constant  over the 128 channel
> cube.  This image is limited by poor phase calibration.  Calibrated with the
> per-integration gain solution, which tracks the detailed phase fluctuations
> much better,  the dynamic range improves dramatically at the band center,
> and deterioriates toward the edges.  E.g., 32 MHz from the band edges, the
> (1 MHz) dynamic range is 1100-1200, and it increases to 1600 at the center.
> Closer to the edges (including where the bandpass drops precipitously), the
> dynamic range drops to 460 at one edge and 700 at the other.  Even at the
> band center, I see no background sources in the 0217 field.


Very interesting...a few questions come to mind.

- Is the change in dynamic range due to increasing noise or decreasing
   signal?

- Are the edge-channel problems antenna- or baseline-based?  One
   could derive antenna-based gains for the edge channels and the center
   channels separately, or look at closure vs. channel.

- A related question, can you see the problem in the visibilites or just
   in the map?  Comparing vector and scalar averaged amp/ph vs. freq
   averaged over all baselines might be instructive, as might rms's in
   both quantities (can CASA plot rms amp/ph vs. frequency???).

Just as an aside, we expect the very edge channels to be sqrt(2) or so
noisier than the middle.  But this should be confined to 10 or so channels
on each end, as reflected in the rapidity of the rise in the bandpasses
at both ends.

Cheers,

                Michael