Intro

    Tsys for cband polB jumped up on 12dec19. It stayed higher than polA until 05jan20 (then we had the earthquakes). When we came back online,  Tsys for polB was still about 3K higher than polA. An oscillation in the system gain was seen on 4jan20.. On 10feb20 it was shown that the dropouts are occurring at the crosshead frequency.

Dec19: SEFD shows tsys jump is real, not a bad cal value.

The plots show cband Tsys PolA, and PolB as well as TsysPolA-TsysPolB for 2019 (.ps) (.pdf)

• Top: Tsys polA, polB.
  
• black is polA, red is polB. The polB jump occurred on 12dec19.
• bottom: TsysPolA - TsysPolB
• the large difference again started on 12dec19.
• The difference is not constant, it jumps up and down.
• This may be a bad connection somewhere...
  

SEFD shows real tsys problem.

• 15dec19 B1005+077 (3C237) tracked
• 22dec19 B0518+165 (3C138) tracked (10% polarized at cband )
  

• Page 1: measured Tsys and SEFD
• The colors show different frequencies: black:4500, red:4860, green:5000 MHz
• + lines are polA
  
• * lines are polB
• The two sources are plotted together.
  
• Top: Tsys:
• The polB values are about 10 to 15K higher than polA
• bottom: SEFD:
• PolB SEFD is higher than the polA SEFD.
• There is also an sefd variation of both pols because the dish surface is still out of alignment.
  
• Page 2: PolA/PolB ratios
• top: TsysA/TsysB
• The average value of the ratio is plotted next to each frequency.
• It is about .6 to .7
  
• Bottom: SEFDA/SEFDB
• The source B1005+077 has ratios around .6 to .7.
• The ratios for B0518+165 is around .5 and increases at low za (where the azimuth swings around).
• This source is polarized so we might have some differences in the ratio from the parallactic angle change (at low za).
  

Dec x111 data shows problem is always present

The plots show the total power from the x111 runs during dec19 (.ps) (.pdf)

• The 25 Mhz bandwidth around 4500 Mhz had 27 scans  during x111 runs in dec19.
• Each scan had 60 1 second spectra. The total power was then computed from each spectrum.
  
• Page 1: total power vs time for each scan:
• Top frame: PolA total power vs time for each scan
• a display offset was added to each scan. the lower line is the beginning of dec
• Bottom frame polB
• page2. rms/mean for each scann
• The rms/mean was computed for each 60 second scan
• Black is polA
• red is polB
• The rms jumped up on 12dec19 and stayed high for the rest of the month.

04jan20: a3123 reports problem with cband

    A3123 reported a problem with their cband observation on 04jan20.

the plots show the total power  vs time for a 60 seconds of data (.ps) (.pdf)

•  7 165 Mhz bands were taken with a .1 second  sampling rate
• the total power vs was computed for each band at the 10hz rate.
• Page 1: total power vs time. overploting the total power from the 7 bands.
  
• top PolA total power vs time
• middle PolB total power vs time
• bottom: blowup of polB total power vs time.
• Page 2: spectra of the total power
• top: polA
• bottom: polb
• multiples of 1.2 hz (the cross head frequency) have been flagged)
• For some reason the power at the fundamental is low.

200210: 2millisec spectra shows dropouts.

    On 10feb20 data was taken with the mock spectrometer to look into the cband polB problem. the setup was:

• 172 Mhz bandwidth, centered at 4500Mhz. 30 seconds of data was taken with 2 millisecond sampling.

• Page 1: total power vs time
• Top: polA
  
• middle : polB
• the y scale is in units of Tsys. The dropouts got down to .5Tsys
• the dropouts lasted from 100 to 400 milliseconds
• bottom: blowup of polB
• Lining up the minimum values gave a period closer to .83 seconds rather than the .8 seconds of the crosshead.
  
• Page 2 : spectra of the total power.
  
• the sample rate of 2ms gave  a 250 hz bandwidth
  
• top: polA
• 60 and 180 hz are sticking up
• middle polB
• a finally spaced comb is visible
• bottom: blowup of polB spectra
• the red lines flag multiples of 1.2 hz (the cold head frequency)

processing: x101/200210/cb_osc_200210.pro

Summary:

• Cband polB Tsys jumped up on 12dec19
• calibration scans in dec19 showed that the jump in tsys was real, not a problem with the cal value.
  
• Looking at the x111 data, the problem was present most of the time.
• a3123 reported a problem with their 04jan20 observations.
• 2ms spectra on 10feb20 showed that the problem is dropouts in the total power  with a period of the cold head (1.2 hz)
• Something is loose in the dewar. When the crosshead cycles, this causes the system gain to decrease.
  

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