[evlatests] Referenced Pointing Problem understood

[Bryan Butler]

Please note that not every observation utilizing referenced pointing is 
affected by this bug.  As I understand it, it only affects observations 
where a reference pointing scan immediately follows a scan on the same 
source.  Most observers do not set up their SBs in that way, aside from 
the possibility of the very first source, so should be immune from this.

	-Bryan


Rick Perley wrote on 9/12/18 10:01:
>      As part of the antenna optics alignment project, my summer student 
> (Lucas Wilkins) and I discovered an important failure mode in referenced 
> pointing.  The details of this are contained in various emails posts 
> distributed last May, and will not be repeated here.    Due to the site 
> power issues, it was not possible to diagnose/correct this problem over 
> the summer.
> 
>      with power now restored, and the array now functional, Barry 
> suggested, and Ken implemented, a 'quick fix' to this problem, which was 
> tested yesterday.  Ken re-ran a Ku-band script used last May, both in 
> the original mode, and with the 'fix' put in.
> 
>      The results are very clear:
> 
>      a) The original script showed the failure mode on every antenna, 
> for every solution.  Resulting solutions (in azimuth) are erratic, and 
> in error.
> 
>      b) The 'fixed' script showed no failures, and the solutions are 
> consistent throughout the six referenced pointing solutions made.
> 
>      Barry tells me that the next step is to devise and test a more 
> elegant solution to the problem, as the current fix is something of a 
> kludge.  I note that the 'quick fix' solution adds ten seconds to every 
> referenced pointing operation.  (The added 10 seconds is a separate 
> scan, with the antennas on source).
> 
>      Until this is done, and the final solution tested and implemented, 
> I strongly urge no observing be done utilizing referenced pointing.
> 
>      Rick
> 
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