[evlatests] Correctness of Switched Power Corrections

[Claire Chandler]

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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