[evlatests] Images from WIDAR, C band

[Sanjay Bhatnagar]

On Mon, Sep 14, 2009 at 8:53 AM, Vivek Dhawan <vdhawan at nrao.edu> wrote:
>
> To try and improve the result on the strong source, individual 4MHz
> chunks were split off, self-calibrated & imaged separately, and images
> co-added. The noise improved as expected.  I think the default option
> on BPASS (which I used) is not optimal. Unfortunately, no other option
> I tried could improve matters sufficiently. I hoped that proper use
> of Bpass would be equivalent to the split-and-selfcalibrate, but I have
> not found the right recipe.

Using CASA, I imaged the same data from Vivek (3C286@ C-Band) with the
flags and delay calibration applied.

Bandpass calibration was done every 60s, and the central channels from
all the 4 subbands were used to make continuum images.  Bandpass
calibration in CASA essentially does independent calibration for each
channel.

Imaging with 4x4, 4x8 and 4x16 channels (central n channels picked
from each of the 4
sub-bands) lead to RMS noise of ~1.0, 0.7 and 0.4-0.5 mJy/b
(corresponding to a bandwidth of 8, 16, and 32 MHz).  Looking at the
residual images gives the impression that the deconvolution has not
left
residuals which are purely noise-like.

> Lastly, I did no polarization calibration, and 3C286 is ~10% polarized,
> so I am a bit surprised that split-and-calibrate is able to get within
> a factor 2 of the floor. I expected closure errors from polarization
> but perhaps they do factor by antenna for a short ~1hr observation.

For a 1 hr. observation of a point sources (relatively small PA
coverage), one could argue some fraction of polarization leakage gains
can factor out as antenna based gains (and get absorbed in the
self-cal solutions).

Regards,
sanjay

-- 
संजय