Pointing model 16, mar05 (CH)

Links to plots:

The input data used to compute model 16 (.ps)  (.pdf)
The model16 fit with residuals (.ps)  (.pdf)
Checking the model by removing a source at a time and recomputing the model (.ps)  (.pdf)
pointing error for the sources taken with model16  (.pdf)

Links to sections:

Data used to compute the model.
Fitting the model.
Checking the validity of the model.
Tracking some sources with model 16

Background.   (top)

    The carriage house pointing model (through feb 2005)  was made from a limited set of data taken in 1997. It only included 4 terms : constant, cos(az), sin(az), and sin(za). It was called model7.  The pointing models that are regularly scheduled are made with a gregorian dome receiver and do not work for the carriage house  (the rail deflections and tilts are different).
    Why a 430 ch model now??
    On 11mar05 a dual beam world day experiment was started. It was scheduled to run though 16mar05. The transmitter failed on saturday 12mar05. After the failure (and before removing the dual beam lock of the rotary floor) the only receivers available were the gregorian and carriage house 430 systems.  The gregorian 430 had a room temp receiver and only 1 polarization (since it was setup for 430 transmission).  That left the 430 carriage house.  A recent modification to the  downstairs if/lo also allowed the carriage house signal to be sent to the interim correlator under computer control.
    14 sources were tracked (though not complete tracks) on 13mar05 and 14mar05 using the heiles calibration scans . The 1.5 and 3 Mhz bandwidth measurements were used to fit the model (the wider bandwidths had some rfi).  There were about 320 independant data points to make the model with. The az, za coverage was not complete  (but that's what we got!!). Model7 was the model installed when the data was taken. The new model created will be model16. It will be valid for the carriage house.
    The observing was done at night and during the day. Daytime observing with the ch worked better than with the dome. When the dome is in use, the tiedowns lose tension during the day when the dome is at high za and passes in front of a tower. This does not happen with the carriage house since it only weight 35000 lbs (as opposed to 200,000+ for the dome).

Data used to compute the model.  (top)

    The data used to compute model 16 (.ps) (.pdf) was taken using model7  (the previous model). Figures 1-5 show these errors. Figures 6 and 7 remove the model 7 correction and show the raw telescope pointing error. All errors are great circle arc seconds. Note that the azimuth positions are from the encoder which is on the dome side of the azimuth arm.
  1. Fig. 1 is the azimuth/zenith angle coverage for the input data.
  2. Fig. 2 is the pointing error (za error top, az error bottom) plotted versus azimuth. This is relative to model 7. The left half of each plot is the southern portion of the dish (northern sources with declination > 18.2 degrees). The right half of each plot is the northern portion of the dish (southern sources). Remember that the azimuth plot is from the encoder which is on the dome side of the azimuth arm.
  3. Figure 3 is the pointing error (za error top, az error bottom) versus zenith angle for the input data. There is a linear ramp in za error of -5.15 asecs/degZa relative to model7. This probably comes from the weight change on the dome over the years. It may also have to do with moving the average height of the platform after the dome was moved on its mounts.
  4. Figure 4 is the za and azimuth errors plotted by source order. The sources are color coded.
  5. Fig. 5 is the magnitude and direction  of these errors plotted versus azimuth and za. 1 tick mark is 10 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. At high za there are some large outliers.
  6. Fig. 6 has the raw az, za errors plotted versus azimuth. The model7 correction has been removed. Model 16 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.
  7. Fig. 7 shows  the same raw errors plotted versus za.

Fitting the model.   (top)

    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 model16 fit with residuals (.ps) (.pdf) are:
za residuals az residuals total residuals [asecs]
mod16 noEncTable 16.33 13.63 21.27
mod 16 with Enc Table 7.52 8.79 11.57
  1. 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. You can see some of the outliers at high za.
  2. Fig. 2 plots the azimuth and za (raw Errors - ( model + encoderTable) ) residuals versus za.
  3. Fig. 3 plots the azimuth and za (raw Errors - (model + encoder table) residuals versus azimuth.
  4. Fig. 4 plots the za and azimuth residual errors by source. B0134+329 (3C48 40Jy) has the largest residuals.
  5. Fig. 5 shows the za, az model residuals plotted versus source declination. Again the largest errors are the last source B0134+329.
  6. Fig. 6 has the residual error plotted versus azimuth and zenith angle. 1 tick mark is 10 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 residuals of 11.6 asecs are not great, but they are a lot better than the 21 asec residuals we started with. More important is that the large systematic errors of over 120 asecs are no longer present. The beam width varies from 500 to 600 asecs so worse case the rms is about 2% of the beam.
    One thing to check is that the pitch, roll, focus is ok for the ch. With the gain approaching 18 K/Jy at za=0 we probably are not off too far. A few of the strips showed a coma lobe where the sidelobes were not symmetric on the  two sides of the source.

Checking the validity of the model.   (top)

    The validity of the model is tested by removing a source at a time from the data set and recomputing the model (.ps)  (.pdf). This was done for all 24 sources in the model.
  • Fig 1 has the model residuals removing one source at a time. 0 is B0017.154, 1=B0134+329...The black line is the total rms residuals while the red it 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  2nd source (B0134+329) makes the largest improvement in the model. This source had the largest fit residuals.
  • 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 large jump at B0316+162 is because there was only 1 measured point. The rms along the track was taken to be the point error at that single point.

  • Tracking sources with model 16

        On 16mar05 in the afternoon 5 sources were tracked with model16 installed. The pointing error for the sources taken with model16  (.pdf) is  show in the plots. The top two plots are za error versus za,az and the bottom two are az error versus az,za. The rms were 12.29,9.77 for a total of 15.7 asecs. The large errors are coming from B0134+329. The model looks like it is not doing a good job fitting the za error at high za. The encoder model for za at high za had to fit some residuals with large separations.
    processing: x101/model/mar05_ch/doall.pro, verify.pro