Lbw cal values measured 28apr16

Links to SECTIONS:
Why the cals were remeasured.
Measuring the high correlated cal using blank sky and absorber
Measuring the other cals on blank sky relative to the high correlated cal.
Diagnostics

Why the cals were remeasured.

• The last measurement was on 28jun12.So it's long over due.
  
• Tsys had been increasing for lbw.
  
• We've also seen an increase in the sefd for lbw.. so it's probably not just the cal values changing.. but to debug this problem, having the correct cal values will help.
  

Measuring the high correlated cal using sky and absorber: (top)

• absorber measurement: morning of 28apr16 09:38
  
• on Sky measurements: morning 28apr16. 10:41 to 10:45
  

    The temperatures used in the computation were:
 

Tabsorber	300K (80.5F avg temp sensor)
Tsky	5 K
Treceiver	from test shack feb03
Tscattered	15 K

    The recording and processing steps were:

• The band 1120 to 1750 MHz was covered simultaneously  using 5 mock spectrometers with 172MHz, 1024 channels each.
  
• The 25 Hz hardware winking cal was used.
  
• Spectra were sampled at 2 milliseconds
• there was one pass through the band for absorber, and 1 pass through the band for on sky.
  
• Each 20 millisecond calOn,Caloff block was averaged (throwing out spectra before after each cal transition)
• The median in each channel (over the 120 seconds) was used as the average value for calDeflection/Tsys
  
• The  1120 to 1750 MHz data  was resampled to a fixed spacing of (172/1024=.168MHz). This was needed since the 5 spectra were overlapped.
  
• A robust fit (8th order harmonic, 1st order polynomial) was done to the ratio data (4001 points). For the sky fits, 4 points about each excluded point were also removed.

    The results of the reduction are:

• Fits to the Average CalDeflection/Tsys  and HcorCal in Kelvins(.ps)  (.pdf):  This shows the average Tcal/Tsys data with the fits over plotted in red.
• Page 1: Tcal/Tsys plots
  
• The top two plots are on the absorber (polA,polB)
• The bottom two plots are on the sky.
  
• The units are Tsys (about 30K for sky and 300 K for absorber).
  
• The fitRms is computed for the fraction of the spectra used in fitting. The rms and fraction of spectrum used are printed on each plot.
  
• The radiometer equation should give: rms=sqrt(2ratio)./sqrt(172bw/1024chan*120secs)=.00032
• The abs plots are  within a factor  of two of this.. close to this, the sky plots are about 6 times larger.
  
• Page 2: The Hcorcal in kelvins
  
• The first two plots show the cal fits in kelvins measured from the Sky, absorber, and the sky, absorber ratio (Y factor). The top plot is polA, the middle plot is polB. The dashed line is the receiver temperature used while computing calSky. (measured way back in 2003).
  
•  The calAbs and calY agree while the calSky is different. The calsky Dc level can be shifted up or down by changing the amount of Tscattered.
  
• The Trcvr curve will have the largest affect  on Calsky.
• The bottom plot is the cal In kelvins from the Y factor.
•  The * are spaced every 10 MHz. PolA is black and polB is red. 
• These are the values that will be used for the high correlated cal.

Notes:

• The cal values measured using Absorber and Y ratio  has always been a bit different than the value measured using just the sky.
• The difference seen in apr16 is larger than that computed back in jun12.
• The  difference can come from the value of Tscattered or Trcvr.
• The new polA difference shows a stronger Frequency dependence that polB (and the jun12) values.
• The Treceiver value that i am using comes from ganesh and lisa's measurements on the antenna test range back in 2003. There was another measurement made in 2007 but ganesh never gave me the values..
• Since we've seen an increase in the SEFD of lbw over the last few years ( SEFD changes jan2006 through jun2016), it might be a good idea to remeasure the lbw receiver temperature to see whether it has changed or  not (especially since we have inserted a new foam window after the old one failed back in may15.
  

Measuring the other cals using sky and the high correlated cal (top)

• hcorcal(on,off)
• hcal(on,off),hxcal(on,off),h90cal(on,off)
• hcorcal(on,off)
• lcorcal(on,off),lcal(on,off),lxcal(on,off),l90cal(on,off)
• hcorcal(on,off)

• Each 10 second set was processed similar to the hcorcal above.
• The ratio (calDifX/caloffX was computed (X is each cal type) for each 10second set and then a robust fit was done (same as hcorcal above).
  
• For each of the "other cals" (hcal,hxcal,h90cal,lcal,lcorcal,lxcal,l90cal)" the ratio (calDifX/calOffX)/(HcorCal) was computed. The hcorcal value used was interpolated (in time) from the 3 sets of hcorcal measured.
• The hcorCalFit in kelvins from the hcorcal measurements (see above) was then multiplied into the other cal fit to give each caltype  in Kelvins.
  

     
  The results of the reduction are:

• OtherCals/Hcorcal and the fits (.ps)  (.pdf):
• These plots show the ratio of the otherCal/hcorcal data and the fits to this ratio.
  
• Page 1: high cals
• page 2: low cals
• top frame polA,bottom from polB
  
• Over plotting all of the cals (in deg K) (.ps) (.pdf). 
  
• Top frame has the high  cals, the bottom frams has the low cals
• The solid lines show polA, the dashed lines show polB.
• The colors show the different cal types.l
• the + on the black plot on the upper frame show the 10 MHz samples used for the cal lookup table.

Diagnostics:

• Over plotting the new and old cal values (.ps) (.pdf)..
  
• The solid lines are the new cals. The dash lines are the old cal values. 
• the gap with no data is the platform painting project.
  
• Top HiCalsPolA: black Diode1 -> polA, Red diode2->polA.
• 2nd HiCalsPolB: black Diode1->polB, Red diode2->polB
• 3rd LoCalsPolA: black Diode1 -> polA, Red diode2->polA.
• 4th LoCalsPolB: black Diode1->polB, Red diode2->polB
• The cal values for PolA and polB  have decreased  by about 20% since the 2012 measurements.
• This is probably from a change in attenuation between the diode and the dewar (or I guess the diodes them selves could  have changed.
• Comparing Tsys measurements using 28apr16 and jun12  cals  (.ps) (.pdf):
• The plots show the tsys measurements jan16 -> jul16
• Tsys is plotted using the old cal values (polA, polB) and then replotted using the new cal values (polA, polB)
• A horizontal dashed lines shows 30K
  
• Top Frame: hcal (diode1 -> polA, diode2->polB)
• PolA (black,red) and  polB (green,blue) have both changed.
  
• 2nd frame: hcorcal (diode 1->polA, diode1 ->polB)
• The results are the same.
• Bottom frame (TsysPolA - TsysPolB).
  
• Black is TsysDif for  the old cals, red is tsysDif for the new cals.
• The receiver was brought down to the lab for maintenance during the painting shutdown.
• when the receiver was reinstalled in apr16, tsysA-TsysB decreased by about 5K.

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