[evlatests] Results from WIDAR K-band test

[Rick Perley]

I missed this.

Indeed, as Steve has reported, antenna 25 (only!) has some very curious 
'drop-downs'.  These affect all subbands and both polarizations 
equally.  They typically last 3 integrations -- 3 seconds.  Phase is not 
affected, except in those where the amplitude drops to the noise (but 
never to zero).  This indicates that this is a real signal loss, and not 
due to some effect of scaling the data. 



Steven T. Myers wrote:
>
> 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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