[evlatests] L-Band Sensitivity tests

[Rick Perley]

    After consultations with Ken and Bob, an experiment was fashioned to 
try shed light on the L-band sensitivity differences repeatedly noted 
between IF pairs 1 and 2.  In short, we have always noted that:

    -- IF Pair 1 (IFs A and C) always have significantly poorer 
sensitivity than IF Pair 2 (IFs B and D). 
Given that we have always used the standard default frequencies, with A 
and C set to 1465 MHz, and
B and D set to 1385 MHz, the difference could be due to a frequency 
dependent antenna feed or Tsys dependency, or due to some IF channel 
dependency. 

    To separate these effects, we observed a standard calibrator (3C286) 
with four different combinations:

    1)  IFPair 1 = 1485, IFPair 2 = 1385

    2) IFPair 1 = 1385, IFPair 2 = 1485

    3) IFPair 1 = 1385, IFPair 2 = 1385

    4) IFPair 1 = 1485, IFPair 2 = 1485. 

    Note that with this setup, we observe each channel twice at each 
frequency. 

    We chose 1485 rather than 1465, in order to avoid the VLA 
Polarizer's 'suckouts' located near
1465 MHz. 

    Observations were made in correlator mode '4', at 6.125 MHz BW, 
giving 32 spectral channels
in all four parallel-hand correlations.  Averaging was set at 5 seconds 
-- as fast as the correlator can go. 

    Calibration used the convenient on-line images of 3C286 (nifty!), 
which removes any resolution-dependent gain variations.    Sensitivity 
was judged via AIPS weights, which have proven to be a reliable 
(although not guaranteed!) metric for sensitivity.   AIPS weights are 
nominally 1/sigma^2, and hence proportional to (efficiency/Tsys)^2.    A 
high value is a good thing. 

    Results:

    First, some observations of stability: 

    Antennas 13 and 18 were regularly unstable -- in a manner exactly 
consistent with the short-term regular dropouts previous described.  
Since these dropouts occur every ten seconds (on the 'ones'), every 
other 5-second integration for antennas 13 and 18 (IFs B and D only on 
18) were low.  These low values were flagged out so as not to bias the 
sensitivity calculations.  We have good reason to believe the unflagged 
values represent good data. 

    All phases were stable over the duration of the scan (about 7 
minutes each). 

    The ~30 second change in gain, at the beginning of the scan, noted 
on Friday for antennas 16 (all IFs)
and 14 (IF D only) were not seen in this experiment -- surely because we 
did not change band. 

    Antenna 18, IFs B and D provided no data at 1485 MHz at either 
observation.  Antenna 13, IFs B and D, provided no data on one 1485 MHz 
observation, but did on the other.  The data were truly dead (this 
determined after filling the data with all flags off) -- not just weak.  
I presume a LO failure. 

    Now, for the sensitivity results.

    In short, the dependency is purely one of frequency.  No clear IF 
dependency is seen (either than
two clearly weak IFs:  14C and 16D). 
    These results are more easily seen in the following table, giving 
the observed AIPS weights (multiplied
by 100).   `X' means no data were obtained (as noted above). 

 IF                                 1385 MHz                           
                                 1485 MHz
-----------------------------------------------------------|--------------------------------------------------------------------
                13            14            16            18      |      
13            14            16            18
-----------------------------------------------------------|--------------------------------------------------------------------
A      |      320         304         310         334       |      150   
      200         200         254
                320         315         310         345       |      
172         215         205         279
--------------------------------------------------------------------------------------------------------------------------
B            305         300         295         350        |      185   
      215         197            X
               340         320         306         370        |      X   
         230         205            X
--------------------------------------------------------------------------------------------------------------------------
C            360         204         280         335        |      165   
       62           185        264
               350         212         278         349        |      
173          79          190         288
---------------------------------------------------------------------------------------------------------------------------
D            300         285          93          340       |      175   
      228             23           X
               340         305         97           365       |         
X         230            21            X
----------------------------------------------------------------------------------------------------------------------------

    The small changes in weight between between the two observations at 
any given frequency/IF
are easily accounted for by the change in elevation of the source. 

    From these results, we see that the sensitivity is significantly 
better at 1385 MHz than
1485 MHz, for all antennas and IFs.   When we swap frequency pairs (i.e. 
observe at 1485/1385,
then 1385/1485 for IFpairs 1/2), the sensitivity moves with the frequency.

    Some IF dependencies are clear:  antenna 16D is clearly in bad 
shape, with either a very
high Tsys, or very low efficiency, regardless of frequency.  Antenna 14C 
is also a poor performer,
but not as severely. 

    Antenna 18 is the best of the four EVLA antennas, at both frequencies. 

    For comparison, here are the *best* VLA weights:

IF      1385      1485
----------------------------
 A      380      405
B       385      410
C       320      330
D      330        290
--------------------------

    From this, we note that in general, the EVLA at 1385 is generally as 
good, or better than
any VLA antenna -- particularly antennas 13 and 18.   And (as noted 
numerous times) at
1485 MHz, the EVLA sensitivity is far poorer than the best VLA (although 
the sensitivity of
18 is about equal to a median VLA antenna). 

    Note that for the VLA, IFs A and B (RCP) are significantly better 
than IFs C and D (LCP) -- at both
frequencies.   This polarization difference is not seen in the EVLA 
antennas. 

    I will next look at the sensitivity histograms, to see where the 
EVLA antennas fall,  and confirm
these results with actual noise histograms, to confirm the reality of 
these conclusions.