[evlatests] Still more on spectral line noise

[Rick Perley]

    All tests reported here were done with 3.333 second integration, 
except for the 780 kHz BW observations -- these were at 6.67 seconds. 

    The constant correlator coefficients indicate that the actual SNR 
remains the same.  The simplest explanation for the oddities at 12.5 MHz 
BW, when in spectral line modes, is that somehow the actual integration 
time is significantly less than 3.33 seconds, or that the actual 
channelwidth is significantly less than the value claimed. 

    I've looked at the sequential visibility amplitudes and phases -- 
they change from record to record in entirely normal ways.  The 
bandpasses look entirely normal. 

    The effect is identical for VLA and EVLA antennas. 

    The noise in the images of the blank field (and of the calibrator 
too!) are completely consistent with the histograms. 

    The earlier PA mode data (taken with 12.5 MHz BW) which revealed 
this problem, included observations of 3C286, which showed normal 
polarized flux, and normal PCAL solutions.  Everything looks normal -- 
except the noise ...

   

Michael Rupen wrote:
> To follow up on George...what integration time was used for these tests,
> and does the noise change with integration time as one would expect for
> the 12.5 MHz modes?
>
>             Michael
>
>
> On Wed, 16 Apr 2008, George Moellenbrock wrote:
>
>   
>> Rick,
>>
>>     
>>> Note that for all observations, the data were
>>> loaded as correlation coefficients, and for all datasets, the correction
>>> required to convert these to flux density were the same.  In other
>>> words, the apparent loss of sensitivity seen in the noisier maps and
>>> wider histograms for the 12.5 MHz data were *not* seen in the
>>> correlation coefficients.  How can this be?
>>>       
>> By "not seen", you must mean that the correlation coefficients are not
>> _reduced_ by this effect, such as an increase in pure Tsys would do.  I
>> thought this was already established by the claim that fractional
>> polarizations come out ok for mode(s) where the cross-hands have the
>> ~correct noise.
>>
>> I presume your SEFDs are the just histogram widths corrected for bandwidth
>> (so you can uniformly compare different BWs)?  This, combined with
>> globally consistent calibration factors establishes that this is not a
>> pure (pre-corr) system noise effect.  Since it is presumably not the
>> bandwidth (bandpasses look ok?), it must therefore be that the effective
>> integration time is actually only some fraction of what it is supposed to
>> be.  Your SEFDs are revealing not just Tsys(Jy), but rather
>> Tsys(Jy)/sqrt(dt), where dt is the effective integration time.
>>
>> Remember, the Tsys(K or Jy) sets the scale of both the noise and the
>> correlation coefficient (and thus the calibration factors you require to
>> reach Jy), but the integration time (and bandwidth) influence only the
>> scale of the noise.  I think the cross-hands in mode PA are the most
>> interesting fact available.  They show that the correct full sensitivity
>> is indeed somehow available for 12.5 bandwidth.  If these cross-hands were
>> also noiser, it would have been much harder to notice this problem. They
>> are the key to understanding it, I think.
>>
>> I would confirm that the fractional pol comes out right in mode PA for a
>> known calibrator (e.g., 3C286), to confirm the above assertions.  I think
>> the pol tests earlier in the year probably already establish this.
>>
>> Then I would concentrate on understanding how most 12.5 MHz bandwidth
>> spectral line correlations might somehow be throwing away ~75% of the
>> integration time (or duplicating 25% of the bitstream by 4X?)  I don't
>> know how the h/w & s/w work.  Is something like this possible?
>>
>> -George
>>
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>>     
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