[evlatests] Antennas 13 and 25

[Ray Ferraro]

	The curious amplitude 'square wave' for ant. 13, IF D, is typical of a 
failed path in a VLA correlator sampler. The problem was observed and 
the sampler swapped. Should be OK now.
	Ray Ferraro.

Rick Perley wrote:
>     The curious amplitude 'square wave', of duration ~70 seconds, and 
> amplitude 25%, which I reported as being seen on antenna 13, IF 'D' at 
> L-band, is occurring at C and X bands also.  (I haven't checked K or Q 
> yet).  In all cases, the effect is not seen in Tsys.  It cannot be due 
> to any subreflector issue. 
> 
>     At X-band, I had two scans on the source, between which was a 
> referenced pointing scan. 
>     For the first scan, both antennas 13 and 25 gave very weak fringes 
> -- barely discernible.  The data were flagged. 
>     For the second scan, both antennas 'came to life' -- but with 
> fringes only half the expected amplitudes.  The statistical weights were 
> low by a factor of four -- as expected if (for example) if antenna was 
> mispointed to the half power point. 
>     The referenced pointing scan (in between) failed completely for 
> antenna 25, and gave a large offset of many arcminutes for antenna 13.  
> We are fairly sure that the subreflector is misbehaving on antenna 13.  
> It may also be giving trouble for antenna 25.   
> 
>     Otherwise, C and X bands look quite good on all the other antennas. 
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