[evlatests] C-Band Sensitivity History

[Rick Perley]

    Emmanuel recovered archive data on 3C286 from over the past three 
years, and determined from these the sensitivity of each antenna as a 
function of time.  His table (with the results I got from data taken two 
days ago) is appended.   Listed are the effective system temperatures 
(Teff = Tsys/efficiency), determined from correlator coefficients, for 
IF 'A'.  Note that these values are *independent* of the Tcal values, 
and represent unbiased estimates of the antenna sensitivity.  The values 
were derived for VLA and EVLA antennas separately (i.e., as sub-arrays), 
so the issues with non-matching bandpasses ('closure' errors) are avoided. 

    Two immediate conclusions can be established:

    1) The EVLA's sensitivity requirements leads to a goal of Teff = 
45K.  No antenna has ever achieved this, although many have been quite 
close (notably in winter time). 

    2) There has been a remarkable degradation in antenna sensitivity 
between last month and this, for most of the EVLA antennas.    The VLA 
antennas seem not to share this trend.  Moisture in the feeds is 
certainly the most likely culprit ...