Lbw cal values measured 10jun05

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.

---

Why the cals were remeasured.

    On 16may05 tsys for lbw started jumping (more info).  After some diagnostics it was decided that something was loose in the lbw dewar. It was warmed up and opened on 01jun05. During this inspection, the polB cal cable in the dewar was found to have a crack where it connected at the inside of the dewar. It was then repaired.
 

---

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

    The temperatures used in the computation were:
 

Tabsorber	300 K
Tsky	5 K
Treceiver	from test shack feb03
Tscattered	15 K

    The band 1120 to 1720 Mhz was covered 9 times on absorber and sky. The ratio (CalOn-CalOff)/CallOff was then computed for the data. Each spectra of 600 Mhz (6144 points) was then fit to an 8th order harmonic and 1st order polynomial (the order was chosen to include the ripples in the caldefl/Tsys spectra). The fit was iterated throwing out points whose residuals were greater than 3 sigma . Whenever a point was excluded, 5 points adjacent to the fit were also excluded .

    After examing the fits,  3 passes in polB (while on absorber) were excluded because of jumps in the 1320 to 1420 Mhz band. A robust average of the  passes was then computed (iterating and throwing out outliers). The average spectra was then fit with the same function. See reducing the cal data for more info on the reduction.

    The results of the reduction are:

• Fits to the Average CalDeflection/Tsys  (.ps)  (.pdf):  This shows the average Tcal/Tsys data with the fits overplotted in red.The top two plots are on the absorber (polA,polB) while  the bottom two plots are on the sky.  PolA has 9 passes and  polB had 6 passes through the receiver band.  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(25e6bw/256chan*3secs*9loops*2hanning)=.0006
The absorber fits match this. The sky fits are about 10 time largers. This is because the fits are not fitting the 1 Mhz standing wave from the dish. This is ok since that ripple should not be in the cal value anyway. You can also see that the radar 1240/1260 still gets in through even on the absorber.
• The Hcorcal in kelvins (.ps) (.pdf).
• The first two plots show the cal fits in kelvins measured from the Sky, absorber, and the sky,absorber (Y factor). The top plot is polA, the middle plot is polB. The dashed line is the receiver temperature used for calSky. The calAbs and calY agree while the calSky is not so close.
• 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 cal.
• The major ripple has a period of about 85 Mhz. It is visible in both polarization's (although the level/phase changes a little). It is there on the absorber and the sky so it is not coming from in front of the horn. If it is a reflection in a cable, then it would be about 1.2 meters long (assuming index of refraction=1/.68). Both of these signals are coming from diode 1. The cal measurements using diode 2 also have a ripple.

• CalDeflection/Tsys for the 9 passes (.ps)  (.pdf).
• The first page shows on absorber for the 9 passes through the receiver band. The top plot is polA while the bottom plot is polB. The spectra have been offset for plotting purposes. The units are Tsys (on absorber is about 300K). For polB  the band between 1320 and 1420 Mhz had some jumps (in strips 0,1,and 3). These strips were not used in the computations.
• The second age shows on the sky  for the 9 passes through the receiver band. The top plot is polA while the bottom plot is polB. The spectra have been offset for plotting purposes. The units are Tsys (about 30K on the sky)
• Fits to the CalDeflection/Tsys for the 9 passes (.ps) (.pdf). This over plots  the fits to each pass (6144 points covering the 600 Mhz.). You can see the jumps in the data for polB on absorber. The absorber fits vary by more than the Sky fits since deltaTsys is a larger fraction of the cal when you are on absorber.

---

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)

   The results of the reduction are:

• The average calValues in kelvins and the fits (.ps)  (.pdf):  The 6 passes have been averaged together and then multiplied by the hcorcal fit (in kelvins). There are 14 plots. 7 cals each with polA and polB. The red lines are the fits to the data. The fit rms's are better than 1% over the full band. The fits are doing a better job of interpolating across the rfi than the older method (averaging over the 25 Mhz band)

 
• Over plotting all of the cals (in deg K) (.ps) (.pdf).  The top plot is the high cals and the bottom plot is the low cals. The solid lines are polA while the dashed lines are polB. There are two sets of lines that follow each other. That is because the same diode always feeds two types of cals (e.g. diode1 goes to polA for hcorcal and for huncorcal). These pairs do not track as well for the low cals. This is probably do to the low value of the cal (increasing the errors). The ripple is in all cals but it is strongest in polA.

• CalDeflectionCalX/calDeflectionHcorcal for the 6 passes (.ps)  (.pdf). There is 1 page for each calType (7 pages). The top plot is polA and the bottom plot is polB. The 6 passes through the freq range are overplotted with an offset. The units for the y axis are TcalHcorcal since each of the cal deflections have been divided by the hcorcal deflection.
• Fits to the CalDeflX/calHcorcalDefl for the 6 passes (.ps) (.pdf). This over plots  the fits to each pass (6144 points covering the 600 Mhz.).
• 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 plots are:
• 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 old cal value were a 3rd order polynomial fit to the 25 Mhz averages. Looking at the input spectra of the old data, ripple was present but not as large as it currently is.  polB cal cable was the one repaired. Diode2 -> polB has a large jump up (since more cal is  getting in) but Diode1 going to polB did not increase as much. That is a bit of a mystery.

home_~phil