Telescope gain during and after surface adjustment (Sband)
14jan01 (last mod 16feb01 for focus gain)
The main reflector surface was measured in oct/nov00.
It was found to be way out of adjustment: rms near 1.2 cm and points
near the southern rim up to 5 cm. The measurements consisted of only
3000 targets situated along the support cables (there were 2 small patches
where targets were place on every panel).
Adjusting the surface started in dec00 and by 08jan01
the eastern half of the reflector had been adjusted and by 13jan01 the
adjustements had been completed (they did the 2nd half of the dish
in 1 week???). This adjustment was to take out the large scale variations
in the surface. Another adjusting cycle will be needed with all 38000 panels
targeted to do the fine scale panel adjustments.
On 04jan01 through 08jan01 a number of calibrator
sources were tracked using the sband narrow receiver at 2380 Mhz and the
standard calibration pattern (4 cross scans at 45 degrees to one another).
This pattern took about 5 minutes to do. Each pattern was then fit
for the main beam with coma and sidelines. The
04thru08jan01 figures show the results of these measurements.
On 13jan01 another set of measurements was done
with the dish completed. The
13jan01 figures show the results of these measurements.
Figure 1 shows the point source gain in K/Jy versus azimuth angle.
The solid lines are sources rising in the west while the dotted lines are
setting in the east. The dotted dotted vertical lines show transit
for northern and southern sources. The fluxes come from chris salters flux.cat
catalog. From the plots,you can see that some of the fluxes are a
bit off. The lower figure shows a different set of sources tracked during
the sep00 calibration run (B2249+185, and B0134+329(3C48) were the only
sources in common).
The figures show that:
The adjustment is working (they're turning the turnbuckles
in the right direction!). It is a bit surprising that the increase for
B2248+185 is greater in the west where the adjustment has not yet been
done (the truth is the reflector adjustements were probably a lot farther
along than "half way")
You need to take some of the gain values with a grain of salt (or shot
of rum..) . In particular B1749+06 in sep00 and probably B0127+233,
B1602+014, and B001+154 look a bit high so maybe the flux is off a little.
B0134+329 (3C48) should be the most reliable.
The adjustment is changing the pointing. Errors of 20+ arcsecs will affect
the gain at higher frequencies. We probably need to redo the focus curves
too. The rms pointing error for all the sources in 11.4 arcseconds. Some
of the source position maybe a bit off since they came from the calibrator
catalog and not the pointing catalog (although in general the source errors
are pretty systematic).
The totalBeamEfficiency/(point source gain) is higher for rising sources
than for setting for za < 15. You don't see this as much for the mainBeamRatio.
Larger side lobes for rising sources are probably causing this.
After correcting for the pitch, roll, and focus errors, the gain
for setting sources (where the dish has been adjusted) is consistently
better than the rising side. After the adjustements were completed,
the only full track shows 3C48 stronger on set.
Figure 2 shows the same point source gain in K/Jy versus zenith angle with
the bottom plot coming from the sep00 run.
Figure 3 plots the gain for B2249+185 and B0134+329(3C48) which were common
for both dates. The gain in K/Jy is shown at the top while the ratio
gainJan01/gainSep00 is on the bottom. The gain increase varies from 15
to 28 % for the 18 degree dec source and 40% for the 32 deg dec source
(rise only). Comparing 3C48 on 13jan01 and 08jan01 the setting values
differ by a few tenths of Jy'.s
Figure 4 plots the great circle azimuth pointing error versus azimuth and
Figure 5 plots the zenith angle pointing error versus azimuth and zenith
Figure 6 shows the mainbeam and total beam efficiencies in the top two
plots. The former is the integral over the main beam while the latter includes
the main beam and the first sideline.
The ratio of the beam efficiency to the point source gain
is independent of the source flux. These are shown in the bottom two plots
(beam efficiency ratio and total beam efficiency ratio).
The beam efficiency ratio above 15 degrees za increases
since the beam spills over and the main beam widens. This causes the point
source gain to fall. The lost power remains within the main beam so the
beam efficiency falls less rapidly.
The total beam efficiency ratio has more scatter
since the side lobes are a function of azimuth and zenith angle.
Figure 7 corrects the point source gain for pitch, roll, and focus errors
using a model that has been constructed.
processing: x101/010104/doit.pro x101/010113/doit.pro for a description
plotting : x101/010104/plotit.pro x101/010113/plotit.pro