[evlatests] widar test data, jun 08

[Vivek Dhawan]

More on Monday's data. Old details are below.

Images with self-cal alone get to about 9000:1, on both Jun 3
and Jun 8th data.

Applying baseline-specific corrections (correcting the closure
errors with BLCAL) works: a single solution for the whole time
for each baseline improves things by a factor of a few, 10min
solutions give another factor of a few, and 2min solutions get
the dynamic range over 200,000:1. The closure corrections have
no obvious pattern by antenna or time. They are present in both
jun 3rd and jun 8th data.

The bandpass was noticed to vary on timescales of a few minutes,
by a 1-2 degrees and a percent or so in amplitude. It looks
like a phase slope (from delay changes) plus a ripple with 2
or so bumps in 128MHz. However, applying a time-variable bandpass
to the data made no difference to the dynamic range - still ~9000
without BLCAL.

Michael is planning to get a 1hr dataset on the same source with
the VLA to check for confusing sources in the field.



-------- Original Message --------
Subject: [evlatests] widar test data, jun 08
Date: Tue, 09 Jun 2009 16:13:07 -0600
From: Vivek Dhawan <vdhawan at nrao.edu>
Organization: NRAO
To: evlatests at nrao.edu

About 1hr on 0217+73 on Jun 08, single subband of 128MHz
(The intention was to write out only 1 subband; 4 subbands
were written but bands 0,3,4 were 0.)

I processed this and a dataset from Jun 3rd, same source,
corresponding subband. The sequence was SETJY=4.0Jy; FRING;
BPASS; CALIB; SPLIT; IMAGR (plus plots vs. t and f).

1. The delays have changed by ~60ns for 3 stations, betweeen
3rd and 8th. But no significant differences were found in
the bandpass shapes other than the delay.

There are still 2 groups of antennas as before 5 in each set,
180deg jumps between sets, none within a set.

2. The dynamic range in the image, averaged across 120MHz was
   ~4000:1 on each day, the later one about 10% worse but this
could be differences in flagging out edge data. This is at
least a factor 20 above the noise (if I got the sum right).
I don't see any abvious signs of structure in the images,
the residuals floor looks a little gridlike perhaps, rather
than pure noise.

3. So, I can confirm that the raw data show nothing wrong, but the
images have higher rms, just as Rick saw. Writing 1 subband and
not 4 out of the CBA makes no difference.

4. The weights vs. time look different on the 2 days, so I tried
imaging with weights off - not much difference. (Incoming wts
were 1.0 for all data before calibration)


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