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Дата изменения: Sun Jan 6 23:24:11 2002
Дата индексирования: Tue Oct 2 07:29:03 2012
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Compare Turret scans with heiles calibration

16apr01

     Turret scans move the turret back and forth in a sine wave while the dome moves +/- 5 arc minutes. Total power data is sampled at 100 hz in pol A and pol B. A baseline is removed and then a 2-d gaussian is fit to the amplitude, widths, and offsets.
    The heiles calibration uses 4 strips offset by 45 degrees from each other. The correlator records the auto and cross correlations every second. A 2-D gaussian is fit to stokes I. The fit parameters are amp,  offsets, side lobes, and coma.
    The turret scan samples much faster but has the drawback that the gain and tsys change as you move the feed off of the optical axis. There is also no side lobe or coma fit (coma could be added but side lobes would be difficult).

    On 21mar01 CTA21 was tracked with sband narrow transit to set using turret scans to verify the pointing model (during the day). On 04apr01 this was repeated using  heile's calibration method for comparison. The plots show:

The black and red are turret scans polA and polB while the green is the stokes I calibration.  The 12 samples for the calibration method had  32  turret scan samples. The pointing errors differ. Part of thist could be from the different fitting methods. The calibration scans fit for sidelobes and coma while the turret scans just fit for the main beam. The coma (especially at higher za where the pitch,roll,and focus errors are larger) could be biasing the turret fits.
    Since the runs were done on different days, it is also possible that the platform angle was different on the two days. This could happen on a hot day if we lost tension in one of the tiedown cables. I checked the tiedown cable tensions and tiedown 4 tension was small but it did not go to zero.  The last plot of figure 2 shows the corner heights of the platform for the two days while tracking the source. There is a large difference in corners 4 and 8. I aligned the data by looking at the laser ranging point where the za was 5 degrees for each day. The alignment error could be because the calibration and turret scans do different za motions (but i'm a little suspicious).

    The Gain/Tsys for polB and stokes I are identical. Pol A G/t is less. The tsys of polA, polB differ by about 5% so the stokes I G/T is still about 5 % higher than the turret scans.

    The za beam widths (middle plot figure 2) for the calib runs and the turret scans agree. You can see the beam width increase as the beam starts to spill over above 15 degrees za.

    The az beam widths (top plot figure 2)  do not agree. The calib beam width is 10% smaller than the turret scan beam width. The turret scan beam width in az is wrong since it should be smaller than the za beam width (the az direction illumination is larger than the za direction illumination).  The turret scan is moving the horn away from the paraxial ray as it scans the turret back and forth. The gain falls off because of the telescope beam as well as the field of view at the horn. When the beam width is computed, the plate scale at the horn is used to convert turret degree offsets into arcseconds on the sky. The value -45 arcseconds per turret degree was used. If this value was off by 10%, the two methods would agree.

(Note: The az,za errors were wrong until 06jan02 when i realized that the definition of the error was different for the turret scans and the calibration scans:  (measured-computed) and (computed-measured). Putting in the factor of -1 really helped the agreement!!!) 

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