Alfa spider scans taken 29nov04

12dec04 (updated 06jan04)
links to plots:
the results of the calibration runs (,ps)  (.pdf)
the beam maps for the 7 beams (.ps)  (.pdf)

    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:

    The plots show the results of the calibration runs (,ps)  (.pdf). Each pixel is coded with a different color.


    The processing steps for the calibration are:
  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.
  •      Beamaps were then made using the results of the main beam and sidelobe fits. For the sidelobes, only the first 6 terms of the fourier fit were used. This smoothes the sidelobe measurements a bit. The plots show the beam maps for the 7 beams (.ps)  (.pdf). The data is contoured with 3db steps (below the peak) out to -21 db. Each page has the beam maps for a particular beam. The central az, za position for the beam map is printed on each plot (-az is before transit and +az is after transit).It
  • The bottTop is pixel 0, bottom is pixel 1. Pixel 0 is symmetric with a first sidelobe level of about -14 db. Pixel 1 has a coma lobe with a peak of about -10 db.
  •     The central pixel 0 is coma free with sidelobes of -14 db. The outer pixels have coma lobes of -10 to -8 db. The coma lobe is always opposite in direction to the feed offset on the array e.g.: pixel 2 is offset in the + azimuth direction on the array, the coma lobe is on the -azimuth offset side of the beam .  Some of the variation in the sidelobe level for a particular beam can be explained by the az,za difference of blockage.

    To do:

        This first set of observations was to get the analysis software running. We now need to:
    processing: x101/x102/newcal/alfa/,