[Difx-users] New to DiFX - issues with test dataset rdv70

[Rebecca Lin]

Hi all,

I'm new to DiFX and have been trying to test the latest DiFX-2.6.2 version
on the rdv70 test set. I've followed the instructions in the README file
but did not get identical results when comparing my final output to the
reference files. I get the following output when comparing the visibility
files:

At the end, 960 records disagreed on the header
After 1848 records, the mean percentage absolute difference is 0.05823633
and the mean percentage mean difference is -0.01528535 + 0.00029177 i

It appears that the reference files were generated prior to an update in
DiFX-2.6.1 that may have affected Mark4 data. I've been using calcif2 (Calc
9.1, I believe) and found that the .im file generated is slightly different
than the reference one. I've also tried using difxcalc (which I think is
Calc 11) and the generated .im file is also different from the reference
one.

When testing startdifx on the relevant reference files (instead of
generating my own .calc, im, .input files etc.), the generated visibility
file still differed from the reference with the following output:

At the end, 0 records disagreed on the header
After 1848 records, the mean percentage absolute difference is 0.05454927
and the mean percentage mean difference is -0.01417088 + 0.00020612 i

Does anyone know why I'm getting different results from the reference?

I also ran into another issue with startdifx - the machine file generated
does not seem to follow the requirement of at least N+2 computers entries,
where N is the number of telescopes. I'm currently passing my own machine
file which seems to work but would like to know if I missed something when
setting up startdifx.

In addition, I'm having a hard time figuring out how exactly mpifxcorr uses
the delay files generated by calcif2. I have a hard time following the C++
source code as I've only worked in Python. Could someone point me to where
the phasing is done - ie. fringe rotation, subsample time corrections,
cross multiplication and accumulation of visibilities etc.? As well, what
are SRC 0 ANT 0 DRY (us), SRC 0 ANT 0 WET (us) and SRC 0 ANT 0 EL GEOM
referring to in the .im file and why is more than once SRC generated in the
.im file when only 1 source is provided?

Best,
   Rebecca
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