[Difx-users] running the example datasets

[Greg Lindahl]

I'm new to DiFX, and recently took a whack at running the example datasets
(which Adam recently mentioned.) My goal was to prove that I'd built DiFX
correctly, and was getting correct answers. I'd love to do this as an
automated test on my correlation cluster.

It wasn't too hard building DiFX and following the instructions in the
rdv70 tarball. After running diffDiFX, there didn't seem to be any clues
for what a good answer was.

This output from diffDiFX looked a bit alarming:

> the mean percentage absolute difference is 22.00246523

After consulting with CK Chan, he suggested that the rdv70 example output
in the tarball was generated using Calc9, and I was using Calc 11.
The "running DiFX" instructions on the wiki were good enough that I was
able to figure out how to use the .im file in the tarball (which, indeed,
was CALC VERSION 1 and not CALC VERSION 11) and the "mean percentage
absolute difference" dropped to 0.0015. Which I still don't know is correct
in an absolute sense.

Another thing that surprised me was the minimum cluster size needed to run
this example.

This is all probably in the "we already knew the documentation was a bit
sparse" category. I'd be happy to volunteer to do some of the writing/test
the instructions, but I'll need some clues about diffDiFX's output.

-- 
Greg Lindahl
Software Architect, Event Horizon Telescope
Smithsonian Astrophysical Observatory
60 Garden Street | MS 66 | Cambridge, MA 02138
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