[evlatests] Strange System Temperature Changes

[Peggy Perley]

Were the feed heaters turned on? Could be condensation on the feed
windows.

Peggy

Bryan Butler wrote:
> water in the feeds, which has some odd elevation effect?  it rained like 
> hell in ABQ last night - don't know about out at the site...
> 
> 
> Rick Perley wrote:
>>     I secured a four-hour observation early this morning (3 to 7 AM) to 
>> further study EVLA polarization behavior.  As usual, I found other 
>> problems, not associated with what I was seeking ...
>>
>>     The observations were of a single source, cycling through four bands 
>> -- L, C, X, and K.  Data quality in general is quite good.  (Details on 
>> what is not will be reported separately).   Observations were made at 
>> high elevation -- about 60 through 82 degrees, with meridian transit 
>> three hours into the run. 
>>
>>     However,  the C-band calibration revealed a very odd, global, change 
>> in ampliutude over the 4-hour period, in that the antenna amplitudes on 
>> ALL antennas (both VLA and EVLA) dropped by typically 3 - 6 % over the 
>> four-hour period.    For some EVLA antennas (11, 16, 17, 23 and 24), the 
>> effect was considerably less.  All VLA antennas showed the decline at 
>> full amplitude.    The changes were smooth over the timerange.
>>
>>     The decline is amplitude is clearly associated with an increase in 
>> system temperature.  All antennas recorded a dramatic rise in Tsys, with 
>> the same functional form as the loss in amplitude -- typically rising 
>> from 30 to 50 K through the time period!  By filling the data as 
>> correlation coefficients, it is clear that the SNR was declining 
>> throughout the time period, and that the Tsys correction mostly -- but 
>> not completely -- made the necessary adjustments. 
>>
>>       I then checked the other frequencies:
>>
>>        a)  No such Tsys variation is seen at L-band.  Everything was 
>> steady. 
>>        b) A similar -- and even larger -- effect was seen at X-band. 
>>        c)  At K-band, there are antennas whose Tsys rose, and others for 
>> which it fell, during the same time frame. 
>>
>>     So what caused this? 
>>
>>        - Weather can be eliminated.  The skies were reported to be 
>> mostly clear, and there was no wind.  Perusual of the 'wunderground' 
>> database shows the site temperature to be steady, and the winds light 
>> throughout.
>>        - It is not a purely elevation effect.  The antennas rose, then 
>> dropped.  Plotting Tsys, or gain, against elevation shows there is no 
>> direct correlation.  If elevation is involved, there has to be at least 
>> a two-hour lag.    This cannot be due to opacity. 
>>        - It's not due to pointing off the source.  Besides not being 
>> able to make the Tsys rise, the effects are much less noted at K-band.  
>> (See below). 
>>
>>        I can only conclude that the antenna system temperature did 
>> change as reported, that this increase in noise power decreased the 
>> correlation coefficients, which were nearly, but not quite correctly, 
>> adjusted by the synchronous monitoring. 
>>     But as to *why* the system temperatures were changing so 
>> dramatically at X and C bands, on a clear, calm night -- I have no clue. 
>>
>>    
>>       
>>    
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