Alfa spider scans taken 29nov04

    Spider scans (heiles calibration scans) were taken with alfa on 29nov04 using the calibrator B1756+134 (3C365, 2.3 Jy at 1420Mhz with 1.2 % polarization (from NVSS)). The setup was 100 Mhz bandwidth centered at 1420 Mhz. Each strips took 60 seconds and covered 6 beam widths. An NVSS source search showed that the closest source above 10 milliJy is 464 arc seconds so there was no problem with confusion from other sources in the field.
    13 patterns were done starting with pixel 0 and continuing on thru pixel 6 and then pixel 0 thru pixel 6 (skipping pixel 1 the second time). It took about  43 minutes to do 7 pixels (6 minutes per pattern). The telescope covered the za range of 16 to 7 degrees za (50 minute before transit to 30 minutes after transit).  For each pattern, a pixel was centered on the source (using the built in CIMA offsets for each beam) and then the spider scan was done. The analysis used the cal values provided by ganesh back on 21apr04 (the newer cal values have not yet been installed in idl).
 

calibration results:

• Fig 1: gain, Tsys, SEFD, and averageBeamWidth. The gain and Tsys are affected by the accuracy of the cal values used. The sefd only depends on the source flux. The values change because the za/az is changing as well as the motion from center pixel to outer pixel.
• Fig 2: coma parameter, first sidelobe height, main beam efficiency, and main beam efficiency + first sidelobe. The coma parameter measures the asymmetry in the main beam. The beam efficiencies depend on the accuracy of the cals.
• Fig 3: The pointing error in azimuth and za. The plots have za error vs az, za and azimuth error vs za ,az. The mean and rms errors were computed over the 7 pixels.
• Fig 4: The alfa rotation angle that minimizes the pointing error. The array was set to 0 degrees in hardware. Offline I computed the offsets for rotation angles -5. to +5 degrees in .1 degree steps. The measured pointing errors for the first 7 measurements were then modified to reflect this new setting. The rms errors in azimuth and za for the 7 pixels were added in quadrature and then plotted versus rotation angle. The minimum pointing error was found at a rotation angle of 1.25 degrees.  This means that when the users set the rotation to 0 degrees, it is actually sitting at a rotation angle of 1.25 degrees.

Beammaps:

1. Fit the main beam and sidelobes for each strip using 1d gaussians (allow coma for the main beam).
2. Remove the sidelobe fit from each strip then do a 2d gaussian fit to the main beam
3. model the 8 sidelobe measurements with a fft of 8 terms.

• Fig 1 pixel0: 2 beam maps taken at a za of 15.2 and 4.9 degrees. The sidelobe level is -14 db. The map taken at za of 4.9 is a bit more asymetric. This may be from the telescope blockage.
• Fig 2 pixel1: Only 1 beam map was taken at a za=13.9 degrees.  The sidelobe level is -10db
• Fig 3 pixel2: beammaps at a za of 12.4 and 5.5 degrees with a sidelobe level of - db.
• Fig 4 pixel3: Beammaps at 11.0 and 6.3 degrees za. Sidelobe levels of -10 and -8 db.
• Fig 5 pixel4:  Beammaps at 9.6 and 11.4 degrees za. Sidelobe level of -8 db.
• Fig 6 pixel5: Beammaps at 8.5 and 8.7 degrees za. Sidelobe levels of -8 and -10 db.
• Fig 7 pixel6: Beammaps at 7.4 and 10.0 degrees za. Sidelobe levels of -10 db.

To do:

• Track one of the pointing model verification sources rise to set with pixel 0. This was never done after the pointing model was made since the calibration scans were not yet working.  This will give us the az, za offsets for the alfa array to 1 or 2 arc seconds. These offsets should be similar to the other receivers if alfa was placed correctly on the rotary floor.
• Track pixel 0 rise to set on a polarized source to do the full stokes calibration. Process the data with the old and new cal values. Compare the deltaG measurement to see the consistency in the relative measurements of the polA, polB cal values.
• Track a non zero pixel rise to set on a polarized source to do the full stokes calibration.
• Track a source rise to set stepping between the beams. Taking more than 1 cycle rapidly would allow you to interpolate between the pixel 0 measurements for pointing error and maybe separate out the pointing model errors from the beam offset model.
• Track an outer pixel at 0 degrees rotation and look at the beam shape. Then track the same source over the same az/za range with a different rotation angle to see how sensitive the beam shape is to the rotation angle. Check that beam 1 beam maps look like beam 2 beam maps when alfa is rotated by 60 degrees (putting beam 1 in the position of beam 2).