[evlatests] pointing run, Sep 9 2005

[Ken Sowinski]

I wrote about the Spet 9 pointing run:

> 2.  low elevation effect, refraction?
> There is a clear deterioration of the elevation solution at low elevations
> which does not occur for VLA antennas.  This is approximately fit by
> sag (cos(El)), but it is clearly not the correct model.  tan(z) does a
> much better job in fitting the elevation pointing offsets and suggests
> that the refraction correction is in error by about 15 percent; the antennas
> pointing too low.  We should investigate feeding current weather into
> the refraction calculation done in the calc wrapper.


Motivated by the obvious refraction error in the elevation pointing
residuals I had a look at what is calculated for both VLA and EVLA.
>From the pointing run data it was clear that VLA antennas pointed
well in elevation while EVLA antennas pointed too low by about 15 percent
of the nominal refraction correction.  I extracted the refraction 
correction code from both the Modcomps and the wrapper around calc which 
performs this calculation for the EVLA and fed them the real weather and 
model weather at the time of the pointing run of the night of September 7,
2005.  Comparing columns A and D below shows that the elevation residuals
seen are easily accounted for by the difference in estimated refraction
caused by using model as opposed to real weather.  The differences between
columns A and B, or C and D, are small and explained by the use of a 
different method to estimate the zenith path.  The method used by John in 
the calc wrapper is likely to be more correct.

It is clear the EVLA will need to find the current weather to feed
to the refraction calculator, at least when we want to do a pointing 
run.  

          temp  dew  pres
Real wx   13.0  9.0  793.
Model wx   5.5 -9.5  790.
Note that the model got the dew point grossly wrong.

The refraction correction, in arc-minutes, calculated in each of four
different ways:
A Modcomp using real weather
B calc using real weather
C Modcomp using model weather
D calc using model weather
El    A      B      C      D
 8   6.55   6.20   5.74   5.46
 9   5.81   5.56   5.09   4.91
10   5.22   5.04   4.57   4.45
11   4.73   4.60   4.15   4.06
12   4.33   4.23   3.79   3.74
13   3.99   3.90   3.49   3.46
14   3.69   3.63   3.23   3.21
15   3.43   3.38   3.01   3.00