[evlatests] "Split pole"

[George Moellenbrock]

Barry-

> Elevation was being calculated at the antenna, azimuth at the center of
> the array.

Could you provide a little context for this statement, for those of us
not following the various evlatests threads in detail?

The reason my interest is piqued by this is that I understood (by what 
means, I forget) that the azimuth axes of all of the vla antennas/pads 
were, by design, nominally parallel, i.e., each antenna points 
_geometrically_ as if it were located at the center of the array. 
Presumably this simplified geometry calculations for the correlator 
(27-fold).  It also means they all have exactly the same parallactic angle 
(not just approximately so, owing to the limited size of the array). 
>From the perspective of instrumental polarization calibration, this was 
mildly disappointing because, if enough SNR were ever available, and the 
azimuth axes not parallel, we might be able to see the differential 
parallactic angle in cross-hand phases on an observation of a polarized 
source at transit (alas, 3C286 might pass too close to the zenith to track 
it reliably through transit).  This would (marginally!) decouple the left 
and right D-terms, as in VLBI.

Parallel azimuth axes also means that the "zenith" vector at pads far from 
the center is not strictly parallel to local vertical, so there may be 
discernably varied refraction at low elevations from one end of an 
arm to another.

-George