[evlatests] Source position offsets versus distance from calibrator

[Barry Clark]

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?

Ed Fomalont wrote:
>      COMMENTS ON SOURCE POSITIONS FROM C0555plus_27sep09 DATA SET
> 
>      October 2, 2009    Ed Fomalont
> 
> 
> EXPERIMENT DESCRIPTION:
> 
>     Phase referencing between 0555+398 with 5 other calibrators within 
> 16 degrees.
>     4.68 GHz, with 4 IF's, each with 256 channels of 0.5 MHz each (the 
> usual)
> 
> EXPERIMENT REDUCTION:
> 
>     Used 0555+398 as phase reference.  Imaged other sources
>     Analysis only of source positions.  Assumed that observed
>       phase center was the true source position.
> 
> QUICK SUMMARY:  All source displacements are directly proportional
>     to their distance from the phase calibrator.  But, the source
>     position dependence over the frequency band varies INVERSELY with
>     frequency!
> 
> 
> COMPARISON OF SOURCE POSITION OFFSET VERSUS SOURCE SEPARATION
>                      From 0555+398 
> 
> SOURCE    FLUX     DISTANCE      POSITION       RATIO OF
>                   FROM 0555       OFFSET      POSITION OFFSET
>                   Sep  angle    Sep  angle         TO
>            (Jy)  (deg)  (deg)  (asec) (deg   DISTANCE FROM 0555
> 
> 0555+398   5.50     0.0   00     0.00   00        --
> 0559+238   0.62   15.93  177     4.28  174      0.0000746
> 0604+442   0.55    4.71   21     1.29   21      0.0000761
> 0625+446   0.18    7.34   48     2.03   52      0.0000768
> 0644+392   0.59    9.53   93     2.73   97      0.0000796
> 0542+498   8.4    10.30  -13     2.85  -22      0.0000769
> 
> FREQUENCY BEHAVIOR
> 
> 0542+498 ALL      10.30  -13     2.85  -22      0.0000769
> 
> 0542+498 IF1                     2.98  -23      0.0000804  
> 0542+498 IF2                     2.93  -24      0.0000790  
> 0542+498 IF3                     2.84  -22      0.0000765
> 0542+498 IF4                     2.75  -24      0.0000742
> 
> 0542+498 IF1 chan   1-128        2.97  -22      0.0000800 
> 0542+498 IF1 chan 129-256        2.93  -23      0.0000790 
> 
>    i.e. source position varies with inverse frequency observed!!
> 
>    Time variability is hard to obtain.  Some indication the the offset 
> ratio decreased by 10% over
>      the five hours.
> 
> 
> CONCLUSIONS:
> 
>  o  Source position offset varies directly with distance from phase 
> calibrator
> 
>  o  Source Position offset / Source Distance from Calibrator  = 0.000078 
> +/- (02)
>     for this experiment (all sources)
> 
>  o  By the way, this ratio corresponds to a delay error of 4cm,
>     similar to Vivek's 0212 analysis reported yesterday
> 
>  o  BUT, different IF's have different position scaling,
>         varying inversely proportional to the IF, channel frequencies!!
> 
>  o  The progressive deterioration of source images with distance from 
> the calibrator is
>     explained by the source position change over the observed bandwidth.
> 
>  o  The above properties do not behave like a simple frequency scaling.  
> For example
>     a frequency offset of
> 
>             4.8 GHz * 0.000078 = 374 KHz +/- 10 KHz
> 
>     would explain the average source position change, but not the 
> position variation over the
>     observed bandwidth.  A simple delay error also does not explain this.
> 
>  o  Position analysis of the 0212 data may suggest how variable this 
> effect is
> 
>  o  Similar experiments, but cycling among difference frequencies in the 
> band, might show how
>     this offset is related to various correlator signal frequencies, etc.
>  
> 
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