Lbw cal values measured 06aug04
Links to PLOTS:
value versus freq from sky and absorber
of hcorcal on/off-1 on sky
levels during hcorcal measurements
of other cals to hcorcal tracking blank sky.
Tsys using the old and new cal values.
Links to SECTIONS:
Measuring the high correlated cal using
blank sky and absorber
Measuring the other cals on blank sky
relative to the high correlated cal.
Has the cal value stopped drifting??
The high correlated cal value (diode 1 going to
polA and polB) for lband wide was measured 06aug04 on the telescope using
technique. The telescope tracked blank sky and used 3 second
cal on/offs. The data was taken with the radar blanker on for both
the sky and absorber.
Measuring the high correlated
cal using sky and absorber: (top)
Tsys for polB started to drift on 24apr04.
It continued to drift until the first of jun04. This drifting was in the
polB measurement. It did not matter whether diode1 or diode2 was feeding
polB. The sefd was measured for polA and polB using an unpolarized
source and they remained constant. So the drift was in the cal measurement.
It was coming from after the diodes. After waiting for polB to stop
drifting, we remeasured the calvalues using sky and absorber.
The Trcvr measured on the antenna test range in
feb03 were used to remove the Trcvr.
The temperatures used in the computation were:
||from test shack
Each frequency band was measured 9 separate times on the absorber and
9 separate times on the sky. For each measurement the calon/off-1 spectrum
was computed. A 3rd order polynomial was fit to this
data to generate the cal values (throwing out any data points greater than
2 sigma on the first iteration of the fit). The cal
Value versus frequency plots show the results.
of calOn/Caloff-1 for the 9 sky measurements shows the spectra
for the on sky measurements. 100 Mhz junks at a time were taken. Pol A
is clean in the 1300 to 1400 Mhz band while pol B has some gain jumps
in this region. The radar blanker was on during this measurement. This
same behavior was seen in polB in mar03. Maybe polB is saturating somewhere
and not recovering??
Fig 1 is the measured cal value in deg K versus frequency. The colors are:
Blue is measured using the sky and absorber ratio. Black is using just
the absorber and red is using just the sky. The Top plot is polA and the
bottom plot is polB. The dashed red line is the cal value measured
at the antenna test range.. The dash green line is the receiver temperature
used (from the antenna test range measurement). The cal value from the
sky does not agree very well with the values from the absorber and absorber,
Fig 2 is a 3rd order polynomial fit to the calAbsorber and calRatio measured
values (it will be used to generate the cal value table). The green crosses
were used for the fit. The dashed red line is the cal values measured on
the antenna test range.
Fig 3 and 4 show the cal values for each measurement (polA, polB). The
top plot has the 9 measurements on the absorber. The center plot shows
the 9 measurements on the sky. The bottom plot is the absorber, sky ratio.
The jumps in the cal value spectra are from rfi.
The final plot
has the power levels during the measurements.
Fig 1 top is the power at the upstairs fiber optic transmitter for all
of the measurements. There were 324 records on sky followed by 324 records
on the absorber. The large dips are at the edges is coming from the filters
cutting off the unused part of the band. The spectral density is probably
still ok (since -40dm is for 200 Mhz bw).
Fig 1 bottom is the power at the downstairs if power meter.
Fig 2 plots the 50 Mhz band (via the power counters) that is input to the
correlator 8 bit a/d converter. Top to bottom is correlator board 1 (first
25 MHz of 100 MHz section) through board 4 (last 25 MHz of the 100 Mhz
section). The level has been divided by the median value (the scale is
linear in power). The jumps at the beginning are larger since the cal is
a larger fraction of Tsys when on the sky (the first 324 records).
Fig 3 plots the 25 Mhz 9 level total power measured via the 0 lag of the
correlator. A 25 MHz digital filter sits between the power counter of figure
2 and this measurement.
The high correlated cal was measured above using sky
and absorber as the hot and cold load. The other cals were then measured
relative to the high correlated cal. Blank sky was tracked and the following
cal sequence was run:
Measuring the other
cals using sky and the high correlated cal from above. (top)
100 Mhz at a time was measured (4 by 25Mhz) going from
1120 to 1720 Mhz. The cal was cycled on/off for 3 secs at each step.
The entire frequency range was repeated 6 times.
The calOn/calOff -1 spectra was computed for
each measurement (giving cal in units of calOff or Tsys). Each of
these 25 Mhz band passes was cumfiltered. This was done on the 6 loops
of the same freq band (256 channels *6 numbers). The total power was computed
and then the ratio of each cal value relative to the hcorcal was computed
(via interpolation of the 3 measurements). These values were used to generate
the cals table (after multiplying by the hcorcal in kelvins). No polynomial
fit to the data was done. The figures
show the other cals relative to Tsys and the hcorcal:
Fig 1. This is the cal value as a fraction of Tsys for each of the cal
types. The top figure has the high cals while the bottom figure shows the
low cals. The lines with * are polB. The cal types where the same diode
feeds the same polarization (polA horcal,hcal.. polB hxcal,hcorcal, etc)
all track extremely well even thought they were different measurements.
Fig 2. The calType/hcorcal. The top has the high cals while the lower plot
is the lowcals. The frequency ripple in the high cal ratios is probably
from the 90 deg hybrid (diode2 ->). The low cal diode1 ratios have a large
sine wave. This is probably coming from the directional coupler used to
generate the low cal from the high cal.
Fig 3. The hcorcal was measured 3 times in each set of 10 measurements.
This plot shows how well the cal on and cal off values repeated. The places
with jumps is coming from rfi.
The daily Tsys measurements showed the initial problem
with the cal values. I took these values and recomputed the value of Tsys
using the new cal values for the time period 4jun04 thru 12sep04. The plots
versus day number of year for this period.
Has the cal value stopped
Fig 1 Top. Tsys versus day number for 09apr04 to 12sep04 using the old
cal values. Black is polA, red is PolB. The + are the uncorrelated cal
with diode 1 feeding polA and diode 2 feeding polB. The * are the
high correlated cal with diode 1 feeding polA and polB. You can see the
initial jump around day number 115 (24apr04)
Fig 2 bottom. Tsys versus day number using the new cal values for 04jun04
(156) thru 12sep04. The blue dashed line is the start of the new cal values.
The green dashed line is when the cal values were measured (06aug04). On
the day of the measurement (219) the polA and polB tsys values are the
same. Before this date there was some difference (you need to ignore the
big jumps that are probably rfi or some source in the beam during the measurement).
After day 219 it looks like the red and black measurements are starting
to diverge a little. The problem with polB cal drifting has probably not
gone away. We will have to monitor it for awhile longer to see how much