[evlatests] pointing run, Sep 9 2005

[Bryan Butler]

once we have the meteorological data (or the capability to get it into 
the CALC wrapper), i recommend we use the refraction correction in ALMA 
memo 366.

	-bryan


On 9/15/05 13:36, Ken Sowinski wrote:
> 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
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