[evlatests] P-band Tcals and antenna efficiency

[Rick Perley]

The memo that Eric and I released last week, dealing with using the noise diodes to accurately calibrate the visibility amplitudes, did not include P-band.

I have spent much of this week determining whether these methods will work at P-band as well (as they should).   The answer is 'YES'!, but there were some problems and revelations uncovered.

For this analysis, I used the A-configuration 'flux density' observations taken Jan 31, 2025.  The high resolution considerably reduces our vulnerability to RFI, as well as avoiding coupling between antennas which is quite a problem in D configuration.

Five antennas had to be dropped from the analysis:   ea17 was out of the array, and ea 01, 08, 14 and 18 had either no switched power measures, or obviously faulty measures.   Two spectral windows (at 264 and 360 MHz) were wiped out with RFI, two others (456 and 472 MHz) are questionable due to excessive RFI.  The remaining 12 SPWs gave good data.

It was quickly found that only seven antennas had values for their Tcals in the Tcal database.  All others had Tcal = 1.  This is not a significant problem to the analysis, as the derived 'sky-calibrated' Tcals are independent of the database values.

So, proceding with the analysis, the derived values of Tcal were all in the 30 — 40 K range.  This is much too high!  There are in fact 16 antennas with Tcal values listed in https://mctest.evla.nrao.edu/rich/tcals/tcal_300MHz.txt.  (In all cases, these values are single-frequency values, measured either at 300 or 400 MHz).  For these sixteen, twelve had good data.  For these 12, I calculated the ratio of the listed (lab) Tcal values to those derived by the sky-calibration technique.  Their average ratio was 0.51 — the average of the lab-measured Tcal values is about ½ those derived 'on sky'.

The discrepancy is surely due to an incorrect assumed value of antenna efficiency.  Eric looked closely into the code, and found that AIPS used a value of 0.45 as the presumed efficiency (I have no idea where that number came from — it seems it used the lowest frequency L-band value).  So this means that — presuming the average Tcal value is close to the correct value — that the actual P-band antenna efficiency (mid-band) is a paltry 0.23 (!).

Eric has adjusted the presumed value of efficiency in the TYAPL program to this number.  I then adjusted the CD table values (noise diode temperatures) to the average - 18K.  With these new values, an updated database was generated by TYAPL.  As expected, these data show the amplitudes corrected to the right values.

These updated Tcal values are generated as a function of frequency.  For the mid-band SPWs, the values cluster around 15 — 25K (as expected,as the new efficiencies are based on the average of 18K).  Some antennas are clearly a bit lower, some higher, by small amounts.  There is a noted trend with frequency - - Tcal values seem to rise with frequency between 280 and 440 MHz, with one notable exception — the lowest two spectral windows (centered at 232 and 248 MHz), always have a much higher Tcal (especially for the lowest SPW).  This is not due to higher Tcals, but are due to a dramatically lowering of the antenna efficiency — which seems to be about 0.10 at 232 MHz.  The trend noted above may well be due to a declining efficiency with increasing frequency.

I suggest we update the Tcal tables with smoothed versions derived from my results.  We could also have AIPS and CASA 'learn' about the drop in efficiency at the lowest frequencies.

Rick


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