[evlatests] Slow Wobbles -- back with a vengeance!

[Rick Perley]

    I reported, a couple weeks ago, that we had baseline-dependent slow 
(periods of seconds to minutes) oscillations in amplitude and phase on 
some baselines.  These showed up in Ku-band data on 3C84 (a strong, 
unresolved object) in A-configuration.  The magnitude of the effect was 
modest -- a few percent and a few degrees.  The effect was seen only on  
IF 'C', and it was clear that it was a non-closing effect (i.e., cannot 
be removed by antenna-based calibration). 
    A subsequent test, done at the same band, with the same source, with 
the same setup, failed to find these 'wobbles'. 

    But ... They're back! 
   
    About ten days ago, we observed 3C84 at Ku and K bands 
(alternating), while still in A configuration.  The duration of this 
test was 6 hours, and the goal is to find our 'ultimate' dynamic range 
-- and to uncover any remaining subtle effects produced by our system. 
    Overall, the data quality is simply amazing.  Closure levels (except 
for what I'm about to report on below) are effectively unmeasureable -- 
likely less than 0.01%. 

    But in reviewing the data carefully, the 'slow wobble' problem found 
earlier is present throughout the entire new dataset.  But since we have 
two frequencies in this long dataset, some new and hopefully useful 
information has been found. 

    1) The strong slow periodicity is seen in both K and Ku bands for 
each baseline on which the effect can be seen.  The periodicity seems 
unrelated to the baseline coordinates.  (This has yet to be proven).  
Some antennas are more likely to show wobbles than others.  There is no 
spatial relationship between antenna pairs showing the effect.   There 
is no apparent relationship with antenna number or f-shift. 
   
    2) For both, the wobbles are only seen in IF 'C'.  One can make a 
case for them being very faintly visible in the others IFs, but the 
effect is at least an order of magnitude lower. 

    3) The wobbles are seen identically in amplitude and phase, with the 
same period.  Viewed in the complex plane, the 'wobble vector' circles 
about the (stationary) visibility vector. 

    4) The effect is not visible on a secondary source that was 
observed.  This source has 1/15 of the flux of 3C84 -- from this we 
conclude that the wobble effect is multiplicative, not additive, and is 
proportional to the correlated power -- not the total system power. 

    5) The effect is strongest by far in subband #2 (#1 in CASA-speak).  
It is also easily visible in subband #1, and faintly visible in subbands 
5 and 6.  The wobbles are contiguous between subbands 1 and 2 -- which 
Ken says is evidence that the baseline boards are not to blame.  (See 
the attached figures). 

    6) The period of the wobbles is shorter at K band than Ku band, 
roughly at the ratio of the sky frequencies.  For the baseline 2 x 23 
(see attached figures), the wobble period at K-band is 20 seconds.  At 
Ku-band, the period is 30 seconds.   The periodicity of the wobbles is 
different on all baselines, varying from a few seconds to a few tens of 
seconds. 

    7) The amplitude of the wobbles is much higher in K-band than in 
Ku-band.  At K-band, the effect is about 5% of the source flux, and 3.5 
degrees in phase.  (pk-pk).  At Ku-band, the effect is about half of this. 

    The latter two characteristics make us wonder what happens at Ka and 
Q bands.  A short test this evening -- if there is time available -- 
might help here. 

    I have attached four plots to show the effect -- amplitude and phase 
for Ku and K bands.  All plots are of the baseline which shows the 
effect most strongly:  2 x 23. 

    About the only thing I'm willing to conclude from this is that the 
effect is not caused by an external agent, such as RFI. 

    Speculations as to origin, or suggestions for further testing, are 
welcome! 
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