[evlatests] Source offset from phase center

[Ed Fomalont]

To add to the confusion, here are the position results from
imaging a source displaced from the phase center.



   MOVING A SOURCE OFF THE PHASE CENTER

   October 2, 2009  Ed Fomalont

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.

POSSIBLE CONCLUSION:

   (U,V) coordinates have been rotated slightly.

NOTE: This effect is independent of that described in the previous
email concerning the stretching of arc lengths between sources---which
does not depend on the (u,v), but the w-term determined in the
corrlator, I think.  The residual phases are not correct.