[evlatests] Post-transit Violent Oscillations in ea26

[Rick Perley]

     I have been reducing two science data runs, taken at X, Ku, K, and 
Ka bands.   The observations are of 3C273 (a strong quasar with about 18 
Jy flux), a nearby weak, unresolved source (< 2 degrees separation), and 
3C286 (located far away).   The observations were taken on April 11 and 
April 22.

     Extraordinary behavior in ea 26 has been seen on both days. By far 
the worst was on the second day (April 22).

     Because 3C273 can be used as a calibrator, I have generated gains 
for all three sources.  The results at K-band, are shown in the first 
attached file (Ea26-K-All.png).   This shows the *antenna voltage* gains 
as a function of time.  The red points are from 3C286, the green points 
from the nearby calibrator (J1224+0330), the vlus from 3C273 
(J1229+0203).  Meridian transit was at 05:40, local time.

     Referenced pointing (done at Ku-band) was separately done for all 
three sources.  For 3C273, a referenced pointing was done on 3C273 
immediately after the calibrator observation (green), and applied to the 
next three scans.

     The plot shows that, prior to meridian transit,  the antenna 
behavior was relatively good.  ('Relatively' means that the blue points 
(3C273) prior to transit are relatively similar).

     After transit, there is a large, and increasing difference between 
the gains for 3C273 and the nearby calibrator, J1224+0330.  Further 
more, the spread in the amplitude gains for 3C273 indicate large, 
short-term changes in gain.  How large? Refer to the next plot, Ea26-K.png.

     This plot shows a 'blowup' of the gains for a single scan. Evident 
is a 2.6 second oscillation in the gains.  This is seen at all bands -- 
EA26-U.png shows the gains at Ku-band from the same run.  The period is 
the same, but the amplitude is much reduced. The scaling is exactly as 
expected for a physical offset.

     The K-band values can be converted to angular offsets:  For the 
wildest offsets (near 07 h, local time), the maximum gain corresponds to 
a 1.0 arcminute offset, the minimum to a 0.7 arcminute offset -- hence 
the antenna is oscillating with a ~20 arcsecond amplitude, and 2.6 
second period.

     Very oddly, the offset is not seen in the calibrator (green 
points).  The reason for this is that the referenced pointing procedure 
has correctly removed the pointing error for J1224+0330, but has not for 
J1229+0203.  (Recall that referenced pointing was done separately for 
both sources).

     Evidence for this is in the referenced pointing solutions. Below 
are the solutions for the two sources, for this antenna, in arcminutes.

Time  El          Az      Source

-------------------------------------

2.9    0.31    0.10    J1224

3.0    0.28    0.20    3C273

5.1    0.18    0.29    J1224

5.2    0.04    0.16    3C273

6.2    -0.16    0.70    J1224

6.3    0.39    0.47    3C273

7.3    -0.34    0.48    J1224

7.4    0.32    0.49    3C273

8.4    -0.04    0.28    J1224

8.5    0.32    0.36    3C273

---------------------------------------------------------

     Agreement for the two sources is good prior to meridian transit, 
after which there is strong disagreement (0.7 arcminutes -- which agrees 
with the offsets determined from the gain solutions).  It appears that 
the observations of J1224+0330 resulted in correct pointing offsets 
(green plotted points are unchanged in amplitude through the 
observation), while those of 3C273 are spectacularly wrong.

Likely, the oscillations contribute to the error in the pointing 
solution -- by why only on 3C273?


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