[evlatests] Solar Shocking!

[Rick Perley]

     A rather nice example of the effect of solar heating on the VLA's 
pointing has been found.  In our recent Cygnus A transient observations 
(taken over the weekend), the sun rose at nearly exactly the same time 
as the source transited.  The weather was clear and calm.  Examination 
of the referenced pointing solutions showed that the array -- as a whole 
-- changed its elevation pointing by about 0.7 arcminutes about an hour 
after sunrise.  The sign of the necessary pointing correction shows the 
antennas were pointing 'down' by this amount -- exactly as expected for 
the heating of the side of the yoke facing the sun when the antennas are 
pointed in the opposite direction.

     The geometry of this observation was such as to maximize the solar 
heating effect.  At sunrise, the antennas were pointing at azimuth = 0 
(transit of Cygnus A), and elevation ~ 80, so the sun , which was near 
azimuth 90 (positive to east) and elevation + 10, was at right angles to 
the yoke,, with no shadowing from the reflector dish.  But, with an 
hour, due to the +40 degrees declination of Cygnus A, the antennas had 
rotated by nearly 90 degrees, so now the yoke is facing the sun nearly 
perfectly.  With the sun at very low elevation as well, the heating is 
maximized, resulting in the dramatic pointing offset seen in the 
referenced pointing solutions.

     Rick