[evlatests] C-Band imaging, etc.

[Rick Perley]

    We wondered at the test meeting whether the odd phase/amplitude 
oscillations seen in the recent data are repeatable (in some time 
reference frame), and also what the difference is between the June 23 
data (shown at the meeting) and the apparently very much better June 17 
data (both taken at C-band). 

    Reviewing the 17 June data shows right away that there is only one 
baseline on which an oscillatory phase is seen (19 x 24).  This baseline 
also oscillated in the June 24 data.  However, the large oscillations 
seen on 1 x 23, 1 x 28, and 19 x 25 in the June 24 data are *not* 
visible in the 17 June data. 

    The only obvious difference between these two sets of data is the 
hour angle:  The earlier data were taken at HA = -4, while the latter 
data were taken at HA = +4. 
   
    I reproduced the noise-limited C-band image from June 17 -- after BL 
corrections were made.  Applying BL solutions to the June 23 data helps 
quite a bit, but does not eliminate the 'lumpiness'.  Time-variable 
solutions were attempted also (15 minutes) -- no improvement beyond what 
is expected as the number of free parameters is raised.  A single 
background source is seen in both databases.  A super-widefield image 
(4096 x 4096) shows no other detectable structure on any angular scale. 

    I made a UV-restricted image from the June 23 data (as the lumpiness 
is definitely stronger on larger angular scales).  This helps a lot, but 
does not eliminate the lumpiness.  Short baselines are more strongly 
affected, but long baselines seem not to be immune.