[evlatests] Results from WIDAR K-band test

[Steven T. Myers]

Ive looked a bit also (havent gotten to calibration yet), a couple
more things:

- our zeroes (in occasional visibilities) are back

- antenna 25 had occasions where it amp dropped for a couple integrations
   at approximately 10sec intervals:

Field 1
23:33:41.0~23:33:44.0
23:44:01.0~23:44:03.0
23:44:21.0~23:44:24.0
23:44:31.0~23:44:34.0
23:44:41.0~23:44:43.0
Field 2
23:42:41.0~23:42:44.0
23:42:50.0~23:42:52.0
23:48:11.0~23:48:12.0
23:48:22.0~23:48:23.0
23:48:32.0~23:48:34.0
23:48:41.0~23:48:44.0

On Wed, 9 Dec 2009, Rick Perley wrote:

>    Michael took some K-band test data on the late afternoon of Dec 3.
> Twelve antennas were operating, observing two sources separated by about
> 6 degrees.  Dual polarization (RR,LL) was utilized, four 128 MHz-wide
> subbands, with 512 channels for each polarization product -- 250 kHz
> resolution.   A total of 30 minutes observing, with 1 second
> averaging.   Scans were 90 seconds long, alternating between the two
> strong (2.3, and 1.6 Jy) sources.
>
>    In general, data quality was very good.  Exceptions are:
>
>    1) Perfectly-zero records are back.  About 0.1% of the visibilities
> are exactly zeros.
>
>    2) About half the antennas had a notable phase jump between the
> first and second scans.  Those seemingly affected are 2, 3, 19, 25, and
> 27.  Perhaps antenna 3 as well.  I flagged the first scan for all
> antennas, to simplify matters...
>
>    3) Antenna 27 was very weak throughout -- amplitudes down by a
> factor of 3 (so it's like we had a constant offset, to the 10 dB power
> point).  I flagged this antenna out.
>
>    4) Antenna 19 was bizarre throughout:  three subbands in LCP (2, 3,
> 4) had ridiculous bandpass shapes -- I flagged all three.  All
> polarizations had phase jumps for the first 6 minutes.  Following this,
> the amplitudes all dropped by about 50%.  Amplitudes and phases were
> stable thereafter. I removed the first 6 minutes, and crossed by fingers ...
>
>    5) Phase stability was very poor, with smoothly changing phases, by
> up to 100 degrees, throughout.  The amplitudes of these phase variations
> clearly increases with baseline length, and there is good evidence that
> the phases connect between sources, and the variations are identical
> between polarizations and subbands -- all are consistent with an
> atmospheric origin.  I hope the weather was truly bad on that evening ...
>
>    I calibrated the data with a flat 6-hour average throughout.  After
> removing the issues noted above, the stability in the bandpasses, and
> between sources is very good.   There is nothing to indicate any
> problems (but -- the sources are not really strong, and the scans are
> short, so sensitivity is not great).  The differential bandpass
> solutions (solutions made for each observations for each source) is dead
> flat -- only noise is seen on all antennas.
>
>    I ran POSSM, to check the connectivity of the subbands, for each
> baseline, for each source, for each scan.  All amplitudes and phases
> connect smoothly -- there are no subband (aka spectral window)
> discontinuities at a level of ~1 percent.  The LCP window of subband 2
> is occasionally displaced by a couple of percent w.r.t. the adjoining
> windows (subbands), but this is seen equally for both sources, and I bet
> it's caused by something fishy with the bandpass solutions.
>
>
>
>
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|:| Steven T. Myers                      |:|  Tenured Astronomer       |:|
|:| National Radio Astronomy Observatory |:|  Ph:  (575) 835-7294      |:|
|:| P.O. Box O, Socorro, NM 87801        |:|  FAX: (575) 835-7027      |:|
|:| http://www.aoc.nrao.edu/~smyers      |:|  smyers at nrao.edu          |:|
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