[evlatests] Speedy Old Antennas ...

[Rick Perley]

A curious flagging effect has been found from these recent P-band data.

The observation required considerable antenna travel between the Moon 
and DA 240 -- the Moon being south of the zenith, and the other north of 
it, at the same RA, near meridian transit.

In investigating the data quality, I noticed that nine antennas *always* 
got on source well before the other 18, as judged by the on-line flags.  
The difference in time between the 'fast' and 'slow' antennas ranged 
from a few, to up to 40 seconds.

The nine speedy antennas are:  04, 05, 06, 10, 12, 15, 18, 20, and 26.  
Seven of these nine have old ACUs, according to the Monday mornng 
summary.  The two 'new' antennas in the speedy list are 06 and 10.

Antennas 4 and 20 were always the first to reach the target source, 
according to the flags.   Antennas 06 and 10 were no different than the 
others in terms of reaching the target.  What is quite remarkable is 
that the other 16 'new ACU' antennas always reached the target source at 
*exactly* the same time -- every time.  How remarkable this is!  Is this 
really possible?

But were they really early on source?  The evidence is mixed:

     -  When going to the strong calibrator 3C147,  the 'early' data 
were certainly fine.

     - But when going to the Moon, these early data were not good.  The 
error level was very low, and not visible in the individual 2-second 
integrations on this weak source.  But the problem was clearly evident 
in the vector sum data (generated via DFTIM).  What is remarkable here 
is that the period of the bad data *always* ended at *exactly* the same 
time that the 16 'new' ACU antennas were claimed to be on source 
(according to the on-line flags).

     Now -- how is this possible?  It would seem that something other 
than antenna position is influencing the flags.  It's definitely not the 
focus -- only one band (P) was utilized here.

     Rick

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