[evlatests] Source position offsets versus distance from calibrator

[Rick Perley]

    Yes, we are fairly (but not 100% yet) convinced that the reported 
frequencies in the headers are off by 1/2 a channel.  I await a short 
test, with a different channel resolution, to confirm this. 

Eric Greisen wrote:
> Barry Clark wrote:
>   
>> I think what Ed is saying is that the frequency used to calculate
>> u,v is wrong by 375 kHz.  Might this be related to the half channel
>> labeling error in AIPS that people were muttering about the other
>> day?
>>
>>     
>
> I believe that the conclusion from that previous discussion was that the 
> data appear to be at a frequency 1/2 channel different than the 
> frequency which was commanded in the observation and which casa and aips 
>   subsequently reported.
>
> Perhaps this all boils down to understanding what the commanded 
> frequency means.  Michael stated that it is the low end of the first 
> channel and that is what casa and aips then see - an integer at the low 
> end of the channel.  If instead some other software in the correlator 
> setup takes that commanded frequency to be the center of the first 
> channel, then the data would differ from the labels.
>
> AIPS will allow one to test Barry's hypothesis...
>
> Eric
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