[evlatests] aliasing, time variability

[Gustaaf van Moorsel]

This is a first report on the time variability of the aliasing dis-
covered recently in narrow-band spectral line observations.  The
observations presented here are 12 consecutive scans on 0137+331 for
a total of 3 hours, BW code=5, mode 2AC, for a total of 256 channels
and 1.5 MHz bandwidth, all at L-band.

The problem is easily seen on EVLA - EVLA baselines; to illustrate
this I have attached POSSM1316.pdf showing the excess amplitude in
the first 100 channels (0.6 MHz) on baseline 13 - 16.  This is an
average over the full 3 hours; in the following I investigate the
time dependeny of the effect.

Continuing to limit myself to baseline 13 - 16, I determined the
average amplitude for each 15 minute scan in 6 adjacent 0.12 MHz
(20 channels) wide frequency intervals.  In other words, the prev-
ious plot is broken up in chunks of time (scans) and frequency.

The results are shown in the attached file aliasing.pdf.  The legend
on the right is in units of MHz, so the top (blue) points show the
average amplitude around 0.14 MHz (the average of channels 13 - 32)
as a function of time in minutes.  The next - pink - points show the
average amplitude around 0.26 MHz (the average of channels 33 - 52),
etc.

The result is that apart from a dip at 80 minutes, the amplitudes are
largely constant.  Next on my list is to check the phase behavior at
80 minutes.  Obviously, this result is for one baseline only - I int-
end to check out one or two other EVLA - EVLA baselines as well.

Comments and suggestions welcome!

Gustaaf



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