[daip] forwarded message from Neal Miller

[Eric Greisen]

Thoghts?


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From: Neal Miller <nmiller at skysrv.pha.jhu.edu>
To: egreisen at nrao.edu
Subject: install help
Date: Tue, 14 Aug 2007 14:51:45 -0400 (EDT)

Hi Eric.

I'm groping around a little bit trying to understand some
things, but have reached the point where I hope I can
explain enough details that you'll have an obvious explanation.
I'm noticing some differences in flux calibration steps
between different AIPS installations.

I just got a new linux box, and installed 31DEC07 AIPS on
it on August 3rd. I ran it through the Y2K Large tests,
and everything seemed to check out fine. As an additional
test, I took a UV dataset I had calibrated and editted on
my older computer and AIPS installation and imaged it on
the new system (FITLD of the data that had been written
with FITTP, then IMAGR with the same adverbs I had used
when I imaged the data on the older system). Again,
everything went fine and the output images were of the
same rms as when I created them on the older system
(31DEC06 installed in late July 2006).

However, I noticed as I reduced more data on the newer
system that the results were a little different than I
anticipated. I've been working feverishly on my AM889
observations, and have reduced about the first half of
the data on the older system. When apparently excellent
data were producing what seemed to be ~20% higher rms
than I was expecting based on reductions done on the
older system, I started more direct testing. Sure enough,
for some reason the flux scales appear different for the
same data reduced using the same procedures and adverbs.
Even so, the difference is less than what I'm seeing in
the reduced images (for example, I might find my phase
calibrator had a GETJY flux of 0.87 Jy on the older
system and 0.84 on the new one, when I've loaded the
same raw data on each machine and followed the exact
same calibration steps).

I have noticed some changes in the calibration tables.
For example, if I copy an FG table created on the older
system over to data on the newer system, the flags
seem to "shift" in time slightly.

I'm more inclined to believe the older installation,
although I know some of that is sentimentality since
I've used it and trusted it for a year. However, data
reduced on that older system have produced results
consistent with independent outside data (for example,
flux comparison with FIRST sources, a reduction of
one day's worth of AM889 data that Ed Fomalont did,
etc.).

Any suggestions or thoughts you have would be most
welcomed.

Thanks,
Neal
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