[evlatests] Accuracy of Switched Power Application
Steven T. Myers
smyers at nrao.edu
Wed Apr 4 17:26:48 EDT 2012
(not sure Stuartt is on evlatests)
Attached is a plot from a C-band run for Polcal on 20120225 using
3C286, showing the antenna voltage gain factors V_12 = Pcorr12/(g1*g2)
after having applied the switched power correction in casa. This is
a well-behaved band (this is the same dataset Stuartt looked at at
X-band) and the values are about +/- 12% in spread ignoring the outlier(s).
I don't have the "sigmas" handy.
Note that the scaling factor of about 0.31 is what you expect for antenna
efficiency of 0.55
sqrt( 0.55 / 5.625 Jy/K ) = 0.313
so we are close to the expected amplitude scale here.
In principle the TPOL0003 data can be used to calculate these at all bands
and track them as we observe these every month or so.
-s
(Note: in Stuartt's RSRO report there is a typo where its says to use
a 4-bit correlator efficiency of 0.32 when it should be 0.932)
On Wed, 4 Apr 2012, Rick Perley wrote:
> I have now reviewed the accuracy of the switched power application
> at all bands. The results are rather interesting, and certainly
> indicate we have some work to do to get this system into a reasonably
> uniform state.
>
> Method:
>
> I used my flux calibrator observations, taken in mid January. The
> model opacities, system delays, bandpass function, and switched power
> were all applied. To avoid the complication of the elevation dependency
> (see below for discussion), I used an observation of 3C286 at 60 degrees
> elevation to calculate the system gains.
> The weather during this run was clear, and winds were light to
> moderate. The time of the observation of 3C286 was 3AM in the morning
> -- about ideal for a reliable estimate. Referenced pointing was
> determined on 3C286 itself. I utilized the publicly available models
> for the gain estimate -- these are not critical as the array was in DnC
> configuration at the time.
>
> For each band, I selected two frequencies to generate gains from --
> one from the 'AC' side, the other from the 'BD' side. For all unflagged
> antenna gains, I drew histograms of the gain distribution. Analysis of
> the dependencies (see Memo 145 for details) shows that if:
>
> a) The antenna ('AIPS') gain is too high, then the applied value of
> Tcal is too low, and/or the applied value of the antenna efficiency is
> too high.
> b) The antenna gain is too low, then the Tcal value is too high,
> and/or the antenna efficiency is too low.
>
> A perfect application means the AIPS gain is 1.0. For most bands,
> the median correction is within 5% of this value (as Vivek and I have
> always claimed). But for most bands, there is a disturbingly wide range
> of values (as everybody else who has looked at this has always
> claimed). A gain correction less than 1 means the visibility values
> are higher than they should be.
>
> For the table below, I give the median correction from all the
> unflagged antenna gains, and a list of especially discrepant antennas.
>
> Band/IF Frequency Median Gain Especially bad antennas
> -----------------------------------------------------------------------------------------------
> L-A 1465 0.95 Antennas 19, 25, and 28
> L-C 1465 0.96 Same as above
> L-B 1865 0.96 5, 14, 19, 25, 28
> L-D 1865 0.95 8, 14, 19, 25, 28
> S-A 2565 1.00 7, 9, 10, 12, 16, 24 28
> S-C 2565 0.98 7, 9, 10, 12, 16, 28
> S-B 3565 0.94 7, 9, 10, 12, 15, 16
> S-D 3565 0.92 7, 9, 10, 12, 15, 16
> C-A 4885 0.96 24
> C-C 4885 0.98 none
> C-B 6885 0.94 17
> C-D 6885 0.94 none
> X-A 8435 0.94 2, 3, 4, 9, 17, 20, 21, 22, 27
> X-C 8435 0.94 2, 3, 4, 9, 12, 20, 21,
> 22, 23, 27
> X-B 11064 0.96 14
> X-D 11064 0.92 17
> U-A 14965 0.96 7, 17, 19, 22, 23, 24
> U-C 14965 0.99 13, 19, 22, 23, 25
> U-B 17422 1.02 17, 19, 23
> U-D 17422 1.01 17, 23
> K-B 22485 1.35 all of them!
> K-D 22485 1.35 all of them
> K-A 25836 0.85 17, 20
> K-C 25836 0.85 17, 20
> A-B 28386 1.03 2, 3, 5, 7, 13, 14, 15, 17, 20
> A-D 28386 1.03 2, 3, 9, 14, 15, 17, 28
> A-A 36435 1.00 2, 3, 8, 9, 14, 15, 17, 24, 26
> A-C 36435 1.00 2, 3, 8, 12, 13 14, 28
> Q-B 43340 1.02 3, 20, 22
> Q-D 43340 1.02 3, 16, 20, 22
> Q-A 48425 1.15 3, 16, 20, 22
> Q-C 48425 1.15 16, 20, 22
> ------------------------------------------------------------------------------------------------------
>
> Comments:
>
> 1) The median value discrepancy (from 1.0) could be due to an
> incorrect efficiency used in the correction procedure, or a repeatable
> offset in the values for Tcal. The lower frequencies all have values
> less than 1.0, indicating the tabled efficiencies are too high, or the
> Tcals too low. The opposite effect is seen at high frequencies (except
> K-band, which is special).
> 2) K-band is different than all others. On the B-D side (22.5 GHz),
> the spread in gains is huge -- from 0.9 through 2.0, with a uniform
> distribution. Such a spread can only be due to incorrect Tcal values --
> I cannot imagine the antenna efficiencies covering such a range
> (especially since it goes as the square of the gain range!). On the A-C
> side, the spread is not as wide (0.6 through 1.2 -- although this is
> fractionally the same), and the median is offset in the opposite way.
> The K-band receivers are all 'old' -- would this have resulted in all
> incorrect Tcal values?
> 3) The high-frequency Q-band median is high. This is likely due to
> either an erroneously high efficiency (most likely, in my opinion), or
> to an incorrect opacity estimate (in this case the calculated opacity is
> too low).
> 4) The list of discrepant antennas was derived by looking at the
> histograms. For some bands, it's hard to discriminate between 'good'
> and 'bad' -- the distribution is broad and continuous. Notably
> non-uniform bands are S, X (likely an old vs. new receiver issue), K,
> and Ka.
>
> Recommended Actions:
>
> 1) The discrepant antennas need to be addressed first. In some
> cases, the discrepancies are extremely large -- even exceeding a factor
> of two (both high and low) in amplitude (a factor of four in equivalent
> power).
> 2) We should confirm the median offsets are due to to incorrect
> efficiencies, and adjust this parameter. (Alternatively, confirm some
> offset in Tcal measures, and adjust this).
> 3) The problem with K-band needs an explanation.
> 4) An automated procedure to generate these gains needs to be set
> up. Emmanuel is doing this as part of the array checkout procedure that
> we are developing.
>
>
>
>
> _______________________________________________
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>
-------------------------------------------------------------------------
|:| Steven T. Myers |:| Tenured Astronomer |:|
|:| National Radio Astronomy Observatory |:| Ph: (575) 835-7294 |:|
|:| P.O. Box O, Socorro, NM 87801 |:| FAX: (575) 835-7027 |:|
|:| http://www.aoc.nrao.edu/~smyers |:| smyers at nrao.edu |:|
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