[evlatests] Some checks of the "reway" effect

[Bob Sault]

There has been some concern that the noise level in EVLA data are 
changing "significantly" over short periods of time (eg a scan). The
AIPS task "REWAY" has been said to show this.

To check this, Michael gave me some data from April - at four 
different C band frequencies and two K band frequencies - observed
over a couple of days. In  short, looking through these data, I did 
not find evidence for short term changes in the noise level.

The checks computed the means of squares of signal-free channels and 
means of squares of differences between channels when there was 
signal. I checked all the polarizations. The results are pretty 
consistent with the theoretical
thermal noise level. For example, see the attached postscript file, which
shows the mean square of channels at 7 GHz for 4 baselines. The 
plots show the mean squarea are flat within the scatter. The three 
line across each panel are a theoretical expected value and 
theoretical plus/minus 1-sigma around that.  A nominal EVLA sensitivity model
was used to determine these (not the data). The plots show the real system
is somewhat more sensitive than this nominal model.  For 64 channels, a 25% 
change in the mean square corresponds to 2 sigma: a 2-sigma change between
integrations would be common.

There are a few caveats:
- The noise level for some receivers was a factors of a few worse
than the nominal sensitivity model (this is what REWAY is intended
to account for!).
- Some of the data had RFI which affected the mean square.
- The first channel for some antennas in some data was bad (I am 
guessing this is a sampler level problem rather than RFI).

Using medians (rather than means) cleaned up the latter two caveats.

Best regards
Bob
-- 
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Phone: +61-3-83447074
Email: rsault at unimelb.edu.au
Web:   http://astro.ph.unimelb.edu.au/~rsault

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