CTA21 gain at lband, sband, and cband.
04sep,2000 (16feb01 fixed focal gain error)
The source CTA21 (J0318+164) was tracked from
rise to set on 22aug00 (lband), 24aug00 (sband), and 25,26aug00 cband.
One minute on/offs were done followed by a ten second cal on/off. The table
1 shows the values used for the computations:
Table 1. values used.
FWHM focus [in]
showing the data and results.
Sefd, tsys, gain, and cal stability Fig 1-4:
The SEFD, system temperature, gain, and cal stability
are shown in figures 1-4. In the plots, polA is a dotted line and polB
is a dashed line. Different frequencies are marked by different symbols.
The average system temperature and gain for za < 15 deg is shown in
the first 2 rows of table 2 below. The cal stability for cband has a linear
drift through the night. It was caused by the temperature changing
after an sband transmitter experiment (see gain
stability vs temp (aug00) ).
Computing the surface error from the gain at multiple frequencies Fig 5-11:
The ruze formula gives the fractional gain caused by
errors in the surface:
fracgain=exp(-(4*pi*delta/lambda)^2) ... delta=rms surface error,lambda=wavelength
The fractional gain is gainMeasured/gainExpected. By taking ratios
of fractional gains at different frequencies, we can eliminate the expected
gain. The ruze formula only deals with errors in the surface. The gains
we measured include losses from pointing errors, pitch/roll/focus (prf)
errors, and errors from the surface. We need to correct the measured gains
for these other losses before applying the ruze formula. To compute
the ratios of gains, the polA gain at each frequency was fit to a
+ a2*(za-14)^2 + a3*(za-14)^3 +
a4*cos(az) + a5*sin(az)
+ a6*cos(2az) + a7*sin(2az) +
a8*cos(3az) + a9*sin(3az)
The functions were then evaluated at the lbn az,za and the ratios computed.
Figure 5 shows the ruze formula fractional gain versus surface error
rms for lband, sband, and cband.
Figure 6 shows the estimated pitch,roll, and focus error for
the dec strip of J0318+164. The data comes from a fit to the tilt sensor
and theodolite of feb-apr00. The focus is in inches and the pitch/roll
units are .1 degrees.
Figure 7 shows the fractional gain from the pitch/roll/focus errors.
Figure 8 shows the measured gain corrected for the pitch/roll/focus
Figure 9 shows the ratio of gainSb/gainLb, gainCb/gainLb, and gainCb/gainLb.
These are the measured gains with no corrections.
Figure 10 has the gain ratios after correction for pitch/roll/focus
Figure 11. The ruse formula fractional gain for each frequency
was computed for surface errors 0 to 7 mm. The ratios of sb/lb, cb/lb,
and cb/lb were then plotted. An inverse lookup was done using the
average values from figure 11 (table 2 last row). The resultant rms surface
error was then flagged with a vertical line.
Using the gain ratios, the rms surface error for the
track of J0318+164 is 5.15 mm. The sb/lb and cb/lb fall within .15 mm of
this average. Table 2 shows the gains before and after. The last
line assumes the surface has been corrected from 5.15 to 0 mm. The lband
gain of 9 K/Jy is too low. If we assume that the lbn cal value is 5% low,
then the rms error increases to 5.7 mm and the final lbn gain would be
9.6 K/Jy. It looks like we need to find a loss mechanism that is
independent of frequency (blockage??).
Table 2. Average Tsys and Gain.
|avg values for za < 15 degrees
|avg Tsys [K]
|avg Gain [K/Jy]
|avg Gain remove prf loss [k/Jy]
|avg Gain remove prf,surfErr(5.15mm) [K/Jy]
plotting : x101/calmrcvr/cta21plt.pro