[evlatests] Correctness of Switched Power Corrections

[Barry Clark]

Another interesting number to know would be to what accuracy do
the switched powers tie you to another observation at the same
frequency, not on the same day, with a flux calibrator.  (I'd
limit it to within the same configuration, I think.)

Claire Chandler wrote:
> Hi Rick,
> 
> The flux density runs are observed in a very special way, with frequent 
> reference pointing scans.  I would like to see an analysis of the accuracy 
> obtained by just applying the switched power corrections for more standard 
> observations, with reference pointing done every 40-45 mins for the 
> target/phase calibrator.
> 
> Until we understand the accuracy this gives we should not tell any users 
> that they can get by without observing a flux density calibrator.
> 
> Claire
> 
> On Tue, 3 Apr 2012, Rick Perley wrote:
> 
>>    At the ECSV meeting this morning, diverse opinions were again
>> expressed (without evidence) that application of the switched power
>> does, and does not, put the resulting amplitudes on a correct scale.
>>
>>    On the one side, Vivek and I have long claimed that the resulting
>> amplitudes are correct.
>>    On the other side, others say that the resulting values are often
>> significantly in error.
>>
>>    To settle this controversy, I have taken the 'flux density' dataset,
>> and examined the gain solutions, using sources whose fluxes (I claim)
>> are accurate to 1 or 2 percent.  In this note, I report only on the
>> L-band results -- later this afternoon (if no other meetings intervene),
>> I'll report on the other bands.
>>
>>    The flux density data were taken in 'wide-band' mode (yes, huge
>> overkill, but you never know when it might be useful).  For this
>> purpose, I extracted the two 64-MHz-wide subbands centered on 1465 and
>> 1865 MHz.  I applied the delays, bandpasses, opacity, and the switched
>> power values (after suitable editing of variant points), then calculated
>> the antenna-based gains, using my three northern sources J0217+7349,
>> J1153+8058, J1800+7828, with fluxes determined from my full analysis
>> against 3C286.  These are chosen to minimize elevation dependencies,
>> which are very important at the high frequencies.
>>
>>    Results:
>>
>>    Both sides of the argument are correct!
>>
>>    1)  At 1465 MHz, the mean amplitude gains are 0.97 and 0.98 in RCP
>> and LCP.  The median values are 0.93 for both.   At 1865 MHz, the means
>> are 0.98 and .96, while the medians are about 0.94.   The significant
>> difference between mean and median tells us there are significant
>> outrider points ...
>>
>>    2) 85% of the individual antenna values are clustered with 10% of
>> each other.  But we have some very discrepant antennas:  ea19, ea25, and
>> ea28 are all at least 30% in error (this is in amplitude -- so the power
>> is the square of this).   The next worst antennas are ea14 and ea13.  It
>> turns out that the largest errors are on the 'positive' side, which
>> skews the averages to be above the medians.
>>
>>    The median values of ~0.94 look pretty good, but remember that this
>> is amplitude, so the 'median baseline' will show a flux density error of
>> the square of this -- or about 12% in error -- too high.  The cause of
>> this offset is I think most likely due to an error in the assumed
>> antenna efficiency, but could also be due to a bias in the determination
>> of the Tcal values.
>>
>>
>>
>>
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