[evlatests] Results from system sensitivity tests

[Rick Perley]

    A short script, run last week under excellent weather conditions, 
allows a quick determination of system sensitivity.  All the usual 
calibration procedures were done.  The SEFD values below are derived 
both from natural-weight images of blank sky, (I used a single 2MHz-wide 
channel from the center of each band) and from histograms of the noise 
in various Stokes parameters.  Except for L and S bands, these give 
exactly the same results.  For L and S bands, I use the histograms, as 
the images are contaminated by the usual problems with uncleaned 
sidelobes from far-away objects (especially notable when in D 
configuration). 

Band         Freq         Requirement          Achieved
-----------------------------------------------------------------
L               1445 MHz    325 Jy                   380
L               1820             325                        440
S               2560             235                        360
S               3520             235                        250
C              4440              245                        314
C               6626             245                       250
X               8300            300                        270
X              11300           300                        260
Ku            13200            385                       260
Ku            16800            385                       285
K              19500            650                       407
K              25500            650                       485
Ka            29500            760                       605
Ka            36500            760                       840
Q              42000           1220                    1250
Q              48000           2760                    2820
-------------------------------------------------------------

Notes:  Except for Q-band, the 'requirements' apply to mid-band.  In the 
Project Book, no attempt was made to account for varying Tsys and 
antenna efficiency within the bands. 

As has long been known, we are not meeting the sensitivity goals at 
frequencies below ~ 6 GHz.  The planned implementation of the new-design 
thermal gap at C-band should improve sensitivity at the low end (4 -- 6 
GHz) by 10% or more.  I understand that a similar improvement at the low 
end of L-band may be expected for the same reason.  At S-band however, 
this improved design is already implemented, so the high noise at 2560 
MHz is in need of an explanation.  We should do some holography scans to 
look at the aperture illumination.