[evlatests] Source offset from phase center

[Ken Sowinski]

On Fri, 2 Oct 2009, Ed Fomalont wrote:

> FROM THE C0555plus_27sep09 DATA SET
>
> The source 0555+398 was observed at the phase center and then
> displaced 60" and 120" north.  The images were made and found to be
> slightly offset from where they should be.
>
> Although these shifts are small, they appear to be associated with
> ROTATIONS, rather than magnification as seen for the sources far from
> the calibrator, but at or near the phase centers.
>
>
> SOURCE           FLUX     DISTANCE      POSITION       RATIO OF
>                         FROM 0555       OFFSET      POSITION OFFSET
>                         Sep  angle    Sep  angle         TO
>                  (Jy)  (asec) (deg)  (asec) (deg   DISTANCE FROM 0555
>
> 0555+398          5.5     0.0   00     0.00   00         --
> 0555+398 (+1')    5.4    60.0   00     0.073 100      0.00125
> 0555+398 (+2')    5.4   120.0   00     0.164 95       0.00136
>
> Since these anomalous offsets are nearly perpendicular to the source
> offset from the field center, they can be viewed as ROTATIONS
> associated with the (u,v) coordinate.
>
> Effective rotation is 0.0013 radians = 4.5 arcmin.

Or about 18 seconds of time.