Intro:

• Tsys: This uses the cal value. Any variations in the cal value getting to the receiver will cause variations in the measured Tsys.
• We've had variations in the cal value.. mainly from removing/reconnecting the cal cables when the receiver is brought done for maintenance (and then not remeasuring the cal value using sky/absorber). So the variation in the Tsys measurement is not always an accurate measurement of the receiver Tsys.
  
• Gain: The gain uses the cal value and the flux of each source used. 
• SEFD: Since this is the ratio of Tsys/Gain, the cal variation cancels out. The main variability is our ability to know the correct flux for each source.

Causes of SEFD variability:

• Tsys changing. Some causes could be:
• above za's around 18deg the beam starts to see the ground radiation.. increasing tsys.
• accumulation of moisture in the horn/window.
• We did replace the vacuum window apr-may15.
  
• bad cables,connections, matches.
• resonances in the omt at the  frequency where Tsys is measured (we've tried to stay away from these).
  
• Our Tsys includes the sky.. so measuring Tsys while the galaxy is in the beam will increase Tsys.
  
• problems with the lna, biases.
• I'm leaving out variations in the cal value since the SEFD cancels this out.
  
• Gain changing:
• above za=15 the gain starts to drop off because of spillover.
  
• large surface errors in the reflectors (hard to justify at 21 cm).
• pointing errors. Probably not a problem since most data was taken with crosses of +/- 3 bmwidths, and then a 2d Gaussian is fit to get the peak.
• Out of focus:
• This is mainly do to the tiedowns or distomats not working..
  
• At high za, when close to a tiedown direction, we can lose tension in a tiedown cable.. causing the horn to be out of focus.
• rain causes the laser rangers to fail.. so we don't know how to adjust the tiedowns
• We've had multiple periods of time when the distomats stopped working...
  
• Using sources with variable flux. We've tried to use point sources that are not variable.

Selection criteria for the data:

     SEFD  data from jan06 through 30jun16 was used to check how the system performance of lbw has changed with time.

• I started with all lbw calibration runs jan06 through 30jun16
• To get rid of outliers, data was then limited to
• za 4 to 12 degrees
• below za=4, we can have problems with the azimuth moving rapidly
• above 12 -15 degrees the gain starts to fall off. We also have trouble with the tiedowns losing tension.
• 8 < gain < 16 K/Jy  ..
  
• Tsys > 23 K
• freq = 1415  or 1666 MHz
• the calibration scans have 2 setups 
• Low setup. freq: 1155,1310,1375,1415 MHz
• High setup: freq: 1415, 1550,1610,1666
• Note that 1415 is included in both sets.
• The SEFD of lbw decreases as the freq increases. So the SEFD of 1666 is normally lower than 1415 MHz.
• We ended up with:
• 1415 MHz: 1546 measurements
• 1666 MHz : 511 measurements

SEFD plots of 1415 and 1666 MHz

• 
  
• Top Frame: SEFD polA and PolB vs time
• black is polA, red is polB
• The vertical dotted lines are the start of each year.
  
• You can see the SEFD increasing toward the end of 2015
  
• 2ndFrame: SEFD polA - polB vs time
  
• The solid green line is 0 Jy. The dashed lines are +/- .25 Jy
  
• For much of the time, it looks like SEFD polA is a bit higher than polB.
• 3rd Frame: Daily Tsys measurements (at 1415 MHz).
• This is done a few times a day (separate from the occasional calibration runs).
• This starts increasing near the beginning of 2015. Part of this may the cal value being incorrect.
• bottom Frame: limit SEFD to PolA,B difference less that .25 Jy.
• The black + show all the sefd data.
• The red * are a subset of the data where sefd abs(polA-polB) is less than .25 Jy
• We still see a rise in the sefd starting around the middle of 2015. So it must be common to both receivers.
• Page 2: SEFD stokesI, all data with source names
• This is stokes I (polA and B combined).
• The vertical dots show the start of each year.
• The colored horizontal lines are the mean SEFD for each year.
• The table at the bottom lists the sources (by color and symbol).
• 2006 - 2014 the mean SEFD varied from 2.5 to 2.7 Jy
• i'm excluding 2012 median value. Looks like it had some bad measurements toward the end of the year.
• SEFD rising.
• toward the middle of 2015 looks like the sefd started to increase
• The increase may have started back in the middle of 2014
• Most years had a few sefd measurements below 2.6 Jy. This stopped around the middle of 2014.
• 2016 shows a definite rise in SEFD... no measurement below 2.9 Jy.
• Page 3: Average SEFD for each source observation by day.
• We typically take multiple SEFD measurements on a source for each session.
• This page has computed an average value for each source by session.
• This will give an unequal weight to sources that only had a few  measurements per day.
• You can see the rise in SEFD  toward the end of 2015.

1666 MHz SEFD plots (.ps) (.pdf):

• Same plots as above, but using the 1666 MHz data
• There were much fewer 1666 measurements (511 vs 1546).
• Page 1: sefd polA and polB, looking at  individual pols.
  
• Page 2: SEFD stokesI, all data with source names:
  
• the sefd rises toward end of 2015 (not many measurements 2015).
• 2016 sefd definitely higher.
• Page 3: Average SEFD for each source observation by day.

Summary:

• SEFD jan2006 through 30jun2016 used
• 1415 MHz : 1546 Measurements
• 1666 MHz : 511 Measurements
• Both freq show an SEFD rise starting sometime in 2015.
• By 2016 it was about 20% higher than the normal value.
• Some things that may have caused this.
• Felix mentioned that the new vacuum window around may2015 might be causing this..

processing: x101/160621/lbwsefd_hist.pro

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