[daip] DBCON

[Amy Mioduszewski]

Dear Subhashis,

Eric asked me to look into your bug report.  I did the same test
that you did and found there indeed seems to be something wrong
with the data that DBCON shifts 360asec.  However not as wrong as
you state in your message:

> This indicates that DBCON has not shifted the phase of the second data set
> properly.  I would like to add here that the map made with TIMERANGE=5 0 0
> 0, 7 0 0 0 does not even show a source. There is some fuzzy emission seen
> near the centre, indicating the phase has been shifted wrongly for
> different uv-values.

The second data set has a day number of 7 so there is no data between
5 0 0 0 and 7 0 0 0, if you set TIMER=5 0 0 0 9 0 0 0 you should see
a source (I did) at the phase center.  However, the source is a bit
messy and the concatenated dataset makes a much higher noise image than
the non-concatenated datasets.  So there is something odd going on.

However I think this may be because a shift of 360asec is too large
for DBCON.  When I tested it for smaller shifts (36 and 3.6 asec), things
seemed to work fine, the noise in the images went down by ~2^0.5 etc..
DBCON does not shift the tangent point, only the phase center.  The
DBCON EXPLAIN file suggests that UVFIX should be used to shift the
tangent point for large shifts. 

Cheers,

Amy