Pointing model 18, feb-mar12
Links to plots:
input data used to compute model 18 (.ps) (.pdf)
18 fit with residuals (.ps) (.pdf)
model by removing a source at a time and recomputing the model
azimuth encoder table results (.ps) (.pdf)
Measuring the constant offset terms for the
other receivers (.ps) (.pdf)
the raw errors and pointing residuals (.ps) (.pdf)
verify model18 using sbn (.ps)
Links to sections:
Data used to compute the model.
Fitting the model.
Checking the validity of the
Azimuth encoder table.
Measuring the constant offsets for
the "other" receivers.
Variogram of the pointing residuals.
Current status 05jun12 model 18A
- The platform beam repair finished in feb11.
- During the repair, the pointing model was updated to correct
for the changes.
- The repair added weight to the corners. Corner 12 had more
weight than the other corners since it included the broken
beam patch (as well as the beam reinforcements). The extra
weight caused a tilt in the platform. The pointing corrections
could correct for this pointing error, but it did not correct
- Azimuth swings using the tilt
sensors were made in may11 and later on 15feb12 to measure
this platform tilt.
- A new set of reference tiedown positions were computed to
remove the platform tilt. These new positions were used for
making the model.
- Data for the new model (model18) was taken 27feb12-04mar12.
- The model data was taken with model17A installed
- The data was taken with heiles calibration scans. 557 patterns
- Data points were excluded if the tiedowns last tension during
- --> the below hasn't been
completed since i haven't done the verification yet (05mar12)
- On xxx evening i installed the model for sband narrow
and started the verification.
- 5 sources were tracked with sbn using model18A. The pointing errors looked ok
- The other receivers can retrack these sources and compute
the avg difference to get the az,za offsets.
- File locations. The online model files are in
- enctbl18A is the encoder table for model 18.
- modelSB is current for model 18 (sband narrow). All other
modelXXX are the old model.
- subdirectory ./feb12 has model18A for all the feeds.
The az,za offsets (except for modelSB) were computed
from the previous model17A
- azZaOffRn= azZaOffRnMod17A - azZaOffSbnMod17A +
- If the offsets haven't changed then this should be
- To install one of these models, just copy it to the parent
directory: cp modelXB .. and then do a trkinit
on this feed.
- subdirectory ./Modelbkup has a copy of all the models for
model15B (the previous model). If the new model doesn't work
just copy the bkdup copy to etc/Pnt/.
- The code that reduced each days runs are in
- yymmdd= 120227,120228, 120229,120301,120303
- The code to compute the model is in
The data used
to compute model 18 (.ps) (.pdf)
was taken using model17A (the previous model). Figures 1-5
show these errors. Figures 6 and 7 remove the model 17 correction
and show the raw telescope pointing error. All errors are great
circle arc seconds.
Data used to compute the
- Fig. 1 is the azimuth/zenith angle coverage for the input
- Fig. 2 is the pointing error (za error top, az error bottom)
plotted versus azimuth. This is relative to model 17. The left
half of each plot is the northern portion of the dish (southern
sources with declination < 18.2 degrees). The right half of
each plot is the southern portion of the dish (northern
- Figure 3 is the pointing error (za error top, az error bottom)
versus zenith angle for the input data relative to the previous
model17. There is a linear ramp in za error vs za. (-1.6 +
.28*za). It is about 4 asecs at za=19. Back in dec07 (when
model17 was built) we had two more compressors than we do at the
model18 time. This is about 500 lbs less wait on the dome for
- Figure 4 is the za and azimuth errors plotted by source order.
The sources are color coded.
- Fig. 5 is the magnitude and direction of these errors
plotted versus azimuth and za. 1 tick mark is 5 arc seconds. At
the bottom is a table of the average magnitude and rms for the
entire dish and computed for every 5 degrees in za.
- Fig. 6 has the raw az, za errors plotted versus azimuth. The
model 17 correction has been removed. Model 18 will be fit to
this data set. Fits to 1az, 2az, and 3az have been over plotted
with the amplitude and phase angle of the maximum. Part of the
1az term is probably a latitude error in the AO position.
- Fig. 7 shows the same raw errors plotted versus za. T
The model is fit to the raw errors. An encoder
table spaced every .5 degrees in za is computed for azimuth and
zenith angle errors and then removed. The final residuals are great
circle errors. The telescope must move in that direction from
the computed position to point at the source.
The model 18
fit with residuals (.ps) (.pdf)
||total residuals [asecs]
|mod 18 with Enc Table
- Fig. 1 plots the residuals versus za for the azimuth and za
errors. The encoder table has not yet been removed. The
computed encoder table is over plotted in red.
- Fig. 2 plots the azimuth and za (raw Errors - ( model +
encoderTable) ) residuals versus za.
- Fig. 3 plots the azimuth and za (raw Errors - (model + encoder
table) residuals versus azimuth. There is more scatter in the
azimuth residuals that the za. The tilt sensor measurements show
a 6az term over part of the dish. The encoder rack gear
for the azimuth also has some runout. It will cause a azimuth
scatter (with a za dependence since these are great circle
- Fig. 4 plots the za and azimuth residual errors by source.
- Fig. 5 shows the za, az model residuals plotted versus source
- Fig. 6 has the residual error plotted versus azimuth and
zenith angle. 1 tick mark is 5 arc seconds. A table of the
average error and the errors every 5 degrees za is at the bottom
of the plot. Also included is the model parameters and values.
The validity of the model is tested by removing
source at a time from the data set and recomputing the model (.ps)
was done for all 33 sources in the model.
the validity of the model. (top)
Fig 1 has the model residuals removing one source at a time. 0
is J530+135, 1=J0745+101.. to 32=J1840+240. The black line is
the total rms residuals while the red is the azimuth and the
green is the zenith angle. The top plot does not include the
encoder table while the bottom plot includes it.
Removing the 15th source J0532+075
makes the largest improvement in the model. Looking at
figure 6 from the previous section it looks like the last
2 points before setting for this source had large errors. Maybe
the tiedowns had lost tension?
Figure 2 plots the mean pointing error and its rms for each
source track that was not included in the model. The model was
evaluated without source i, then the mean and rms of the
pointing model along the az,za track for source i was computed.
the 16.6 dec source J0521+166 had
only one point (so there is not rms)
the 27.8 dec source (J0519+277) did
have large az residual errors.
An azimuth encoder table for azimuth residuals
was built by smoothing the great circle azimuth residuals in azimuth
and then removing this from the (model-zaEncTbl) azimuth residuals.
I first tried smoothing the little circle errors (azErr/sinza)
thinking that the azimuth encoder wrack gear was the largest culprit
and it should give a little circle error. The residuals didn't get
much better. The low za errors were messing up the averages. This
must mean that the azimuth residual errors are great circle and not
The table step has 1 degree steps in azimuth.
Different az smoothing was tried. The az
encoder table results (.ps) (.pdf)
are shown in the figure: (the azimuth encoder table has not been
- Fig 1 top is the azimuth encoder table made by smoothing to 1
through 6 degrees azimuth (bottom to top)
- Fig 1 bottom plots the azimuth encoder residuals (black line)
for azimuth smoothing 1 through 19 degrees. The green line is
the azimuth residuals without the azimuth encoder table. The red
line is the total residuals (za plus az) for the various
- Using an azimuth encoder table smoothed to 10deg za
would decease the az error from 4.1asecs to 3.2 asecs.
- Fig 2 over plots the azimuth residuals and the az enctable
smoothed to 3 and 6 degrees azimuth.
- Fig3 is a Fourier transform of the azimuth encoder table
(built with 1 degree smoothing). The top plot is plotted versus
cycles and the bottom plot versus period (in degrees). The power
is at 4 cycles and 12 cycles (90 degree spacing and 30 degree
spacing). (I think the az encoder rack gear has 15 degree
az rack gear)
the constant offsets for the "other" receivers. (top)
Data sets used:
|sbn verify sources all nighte
verify run using 07may sources
ran with model17a but tietest..
using sbn sources.
|alfa- B0758+143 .. redo model
verify from 15th
The model includes
constant terms (great circle) in azimuth and zenith angle for each
receiver. These terms can differ receiver to receiver because of
positioning error of the horn on the rotary floor. The model is made
with sband narrow. After the model we need to compute what the
constant offsets for the other receivers are (ideally it would
On 7,8,12,5,21 may12 5 sources were
tracked with sbn and the new model 18 installed. The sources
Some of these sources were then retracked with the other
receivers to measure the receiver constant az,za values from the
sbn model. Before each receiver was used, an offset was included
to get the receiver close to where it was supposed to be.
- 07may12: sbn tracked sources
- 21may12: alfa(B0758+143..redo).
show the tracking error for sbn and the other receivers. (.ps)
The first 11 plots show the sbn error and the other receiver
error (one per page). Black is the sbn measurement. Red, green,
blue, purple are the up to 4 frequency bands of the "other"
receiver. The left column has azimuth errors while the right
column is za errors. The numbers printed are the
mean(sbnErr) - mean(rcvrErr) in arcseconds. The figures are:
Ideally the (sbn-rcv) value should be the same
for all sources and frequencies of a receiver.
- 327 MHz. 3 frequency bands,
- 430 MHz . 3 frequency bands ,
- 800 MHz . 1 frequency band, 2 sources.. The large za offset is
because i put the 75 Arcsec za offset in backwards when updating
the model file for the verify. I removed this error before
actually computing the real offset.
- lbw. 4 frequencies, 2sources.
- sbw. 4 freq, 1 source
- sbh. 4 freq, 2 sources
- cband. 4 frequencies, 2 sources.
- cbandHi. , 4 freq, 2 sources
- xband. 4 frequencies,2 sources
- alfa 2 sources, bm 0. the Za errors for B0758+143 varied
between the first and 2nd day of this source.
- Mean(sbn)-mean(rcv) for each source and frequency band. black
is the azimuth error and red is the za error.
The offsets for the individual receivers as
calculated from the above data is shown in the table below.
model 18 receiver offsets.
||za offsets asecs
the raw errors and pointing residuals (.ps) (.pdf)
shows the correlation of the measurements versus separation of the
points. The residual error and raw pointing error difference is
computed for all points on a pair wise basis. A metric is then
defined for the point separation and is used to bin the data. The
variance of the pair differences for each bin is then computed and
plotted versus the distance. For each figure the top plot is the
pair wise difference of the pointing residuals (including the
zaencoder table) while the bottom plot has the pair wise difference
of the raw errors input to make the model.
Variogram of the pointing
This data can be used to interpolate the
residuals onto an az,za grid (it gives the nugget (y intercept),
range (where the variance increases), and the sill (value
where the variance levels off) for the krigging routine)
- Fig. 1 is the variogram using the great circle angular
separation of the points as the metric.
- The separation was binned to .3 degrees steps.
- The za correlation increases until za=2. degrees and
then levels off
- The az residuals variance increases till about 5 degrees.
The 25 foot spacing of the north south main cables is about
1.6 degrees (1.5 degrees is close to the 25 foot spacing of
the main cables ). The large correlation in the bottom plot is
the 1az term of the raw pointing errors.
Fig. 2 projects the points into the xy plane and then measures the
distance (since the kriging would be done in this plane). It looks
the same as that of figure 1.