[evlatests] Correctness of Switched Power Corrections

[Rick Perley]

    I've done this, and the results are very encouraging.  The 'flux 
density' run had observations spanning over 40 hours -- with a ~6 hour 
gap after the first 30 contiguous hours.  The system gain determined in 
the detached piece lines up beautifully with the initial run -- within 1 
percent at the low frequencies. 

    Rick

Barry Clark wrote:
> 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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