Measuring the sband wide cal values.
The sband wide receiver was mounted on the antenna
range and its receiver temperature (amp and omt) was measured. It was then
mounted on the telescope and the cal values were measured using absorber
and sky (see measuring
the cals on the telescope using absorber and sky for a description).
The correlator configuration used was 25 Mhz
by 256 lags by 4 sbc. 1800 to 3100 MHz was covered in
100 Mhz chunks (13 steps). At each step a 5 second integration with the
cal off was followed by a 5 second integration with the cal on. The frequency
range was covered 3 times (each pass separated by about 6 minutes).
The high correlated cal was used (diode 1 feeds both polarizations).
For the computations we:
The cal value was computed 3 separate ways:
used 6 K for the cold sky temperature.
measured the absorber temperature with a thermometer : 303 K.
used the receiver temperature measured on the test range.
used 14 K for the scattered radiation into the horn.
The rfi in the spectra was masked out before computing the power
in each 25Mhz band. The rfi situation can be seen in the plots
of spectra versus frequency for the range 1800 Mhz to 3100 Mhz. These
are the spectra taken for the cal off. Black is on absorber and red is
on the Sky. The frequencies with lots of rfi are:
1875 - 2100 Mhz
2300 - 2350 Mhz
2400 - 2600 Mhz
Using just the cal on/off on the absorber. This needed the measured receiver
temperature and it assumed that the absorber noise getting into the amp
was 303 K.
Using just the sky. This used the 6 K sky, 14 K scattered radiation, and
the measured receiver temperature.
Using the hot (absorber) and cold (sky) loads to compute the
cal. This needs the absorber value, sky value (cold sky + scattered radiation).
The receiver temperature is not used since it cancels out.
The rfi is so strong at 1940 Mhz (cellular phones) that even the absorber
(black lines) is full of rfi. After removing the rfi, the rms/mean of the
3 separate cal measurements at each frequency was computed. If it was greater
than .015, then that frequency was not used.
The sband wide receiver also has resonances in the OMT. These are visible
in the above plot and are discussed in resonances
in the sband wide feed.
After removing the points with rfi, the cal
values versus frequency were plotted in the figures:
Figure 1 plots the cal value versus frequency. The top plot is PolA and
the bottom plot is PolB. The 3 separate measurements at each frequency
are overplotted. The colors are:
Figure 2 plots the receiver temperature versus frequency measured on the
test range. PolA is black and polB is red. The dashed line is a 2nd
order fit to the measured data. The value at 2580 was not used since it
lies on a resonance in the OMT.
The 3rd order polynomial fit versus frequency only used
the absorber and absorber/sky ratio values. The sky only values were
not used since there were few of them and they jumped around. You
can also see a 200 Mhz ripple in the data. This is probably in the directional
coupler for the cal value. It is small enough that I didn't bother
to try and fit it.The 3rd order polynomial fit of cal value in Kelvins
versus frequency in Ghz is:
Red: the cal value computed using just the absorber.
Green: the cal value computed using just the sky.
Blue: the cal value computed using the absorber and sky (y factor).
Black: the old cal values from 1999.
Blue: a 3rd order polynomial fit to the absorber (red) and ratio
(blue) cal values versus frequency (the green sky only cal values
were not used).
calA[Ghz]= 18.710434 - 9.4532671*f
+ 3.4901421*f^2 - .4608357*f^3
calB[Ghz]=-3.1919432 + 30.9665108*f - 14.7711287*f^2 + 1.8870836*f^3
The polA cal values are flat (~ 1 K) while the polB
values vary by 8 K over the frequency range. The difference is probably
the coupler used. PolA has a new coupler that is not painted while polB
has an older coupler that is painted (the couplers are in the dewar). You
can see that the old cal values both had large slopes (and they were
To check to see how well the absorber was matched
to the horn, we moved the absorber 3 inches and then 6 inches below the
horn. At each position we measured the cal/TsysAbsorber. It was done 3
separate times at each position. The figures
shows the affect of moving the absorber. The variation in the values
increase as we move the absorber from touching the horn to 6 inches away.
To do this test someone actually held the absorber 3 and 6 inches away
(so it may not have been that stable).
processing: x101/010920/sbwcal.pro, sbwres.pro, movabs.pro