[daip] Re: opacity corrections - VLBA data

[Craig Walker]

On 2002.07.17 05:40 VLBI Gast wrote:
> Dear Craig,
> 
> I'm Lucas Lara, writing you from the MPIfR. At present I'm reducing VLBA data 
> at 22 and 43 GHz and trying to derive the opacity corrections for these 
> frequencies. My data have the WX tables attached, with opacity information at 
> both frequencies, as I have seen with the AIPS task 'WETHR'. However it is 
> not clear to me how this information can be read and converted into a SN 
> table for data calibration. Task 'APCAL' does not allow you to read the WX 
> table, and asks for an external ascii file (I'm using AIPS release of 
> 31DEC02). Is it there a way to do this?
> 
> Thank you very much for your help,
> With best wishes,
> 
> Lucas
> 
> 

The opacity corrections are derived by APCAL based mainly on the system temperatures
in the TY table.  APCAL does use weather station data to derive a "ground" temperature
which helps relate Tsys - Treceiver to opacity.  It gets that data from an external
weather file in the format provided in the weather section of the ascii cal file from the 
VLBA (can be found in the same areas where the schedules are put on the NRAO computers).
If that file is not provided to APCAL, it assumes a default 20 deg C (which seems rather high)
As far as I can tell from a quick look through the code, it does not read 
the WX table even if present.  Obviously, that should be on the wish list for 
future enhancements.  Note that APCAL does often have problems fitting for receiver
temperatures so you may need to look at the Tsys vs sec Z plots and provide forced
receiver temperatures.

I think WETHR just displays the weather data and some parameters estimated from the
weather data like opacities.  It can be used to flag data but I don't think that it can
be use for calibration for opacity.  Anyway, I suspect that the Tsys method of deriving opacity
is better than just using the ground data, but I have not compared the results.  Tsys
would be sensitive to anything along the line-of-sight (even trees).

Craig


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